THE SKIN AND THE SENSE-ORGANS.
The general characters and the colouring of the skin have already been described (pp. 4 to 7). The skin consists of the cutis vera or corium, and the cuticle or epidermis, and is possessed of numerous glands.
Fig. 228.
Vertical section through the epidermis from the head of Rana esculenta; after F. E. Schultze. Magnified 600 times.
| A | Horny layer. |
| B | Middle layer. |
| C | Deep layer. |
| D | Isolated mucous-cells. |
a. The epidermis (Figs. 228, 230) consists of several layers of epithelial cells, those of the deepest layer being more or less columnar in form, those of the middle layer shorter and polygonal, while those of the superficial layer are flattened, very transparent, and horny.
All the surfaces of these cells are serrated, the serrations being, however, with difficulty seen on the free surface of the superficial cells (Leydig). Each cell has a distinct nucleus; in the deeper cells the nucleus is oval, broad, and rounded, in the superficial cells flattened and thin.
Fig. 229.
Vertical section through the skin of the back; after Wiedersheim.
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The surface of the epidermis (Fig. 230) forms a very beautiful mosaic of flat cells, chiefly hexagonal in form, with pale, central, oval nuclei. Here and there two adjacent cells appear to enclose a semilunar space (Fig. 228 D); these spaces are occupied by peculiar cells (goblet-cells, E. Schultze; mucous cells, Leydig), which do not belong to the superficial cells but the layer of cells immediately beneath the horny layer (Schultze, Pfitzner). The cells are rounded or flask-shaped, and closely resemble the epithelial goblet-cells or chalice-cells. According to Rudneff they open on the free surface by stomata; this is, however, denied by Schultze and others.
According to Pfitzner these cells secrete a substance, which has an important function in connection with the process of casting the skin, which consists in the separation of the upper layer from that below brings about a complete shedding of the skin (Wiedersheim).
Fig. 230.
Surface view of epidermis of Rana temporaria; after Eberth.
| A | Epidermal cell. |
| B | Stoma-cell. |
| C | Attached cuticle from duct of gland. |
| D | Protoplasm and nucleus of stoma-cell. |
Fig. 231.
Nerve terminations in the branched pigment-cells of the cutis; after Ehrmann. Hartnack, Obj. 8, Oc. 3.
| I. | Pigment ceasing abruptly at junction of nerve. |
| II. | Pigment gradually ceasing along the nerve. |
[The horny layer is, for the most part, very thin, as it consists of one or two layers of flattened cells only (Schultze), but in some situations, as on the back and especially on the under surface of the toes, it is much thickened, and is then rough.
Deeply pigmented, branched cells, capable of contractile movements, are also found, somewhat sparsely distributed, in the epidermis (Leydig, H. Müller, Schultze).]
b. The cutis (Fig. 229, Co, Co1, Co2). The epidermis is generally attached to the corium by means of a continuous layer of branched cells, which is deeply stained when the animals are fed with madder (Katschenko). Many of these cells are pigmented. This layer is seldom flat, but is raised into papillae and folds, which are repeated by the superimposed epidermis. In addition to this layer the corium has, except in the webs and supplemental toes, three distinct layers of connective-tissue, together with much unstriped muscle-fibre (Eberth).
The superficial layer (Fig. 229 Co) is a loosely-meshed, much pigmented, vascular layer; it forms a loose support for the numerous glands, and is traversed by numerous nerves.
The middle layer (Co1) forms the groundwork of the cutis; it is much firmer and more compact than the superficial layer; in section it appears as a broad band, bounded superficially by a sharp line. It is chiefly composed of closely packed connective-tissue fibres, which have, for the main part, a horizontal or wavy course; at certain points, however, vertical fibres are seen passing towards the surface and hiding the sharp contour (between Co and Co1). At such points the two sets of fibres form a sort of basket-work arrangement.
The deepest layer (Fig. 229 Co2) is composed of very delicate, white and yellow elastic fibres, and vessels and nerves; it attaches the whole integument to the underlying organs. By the looseness of its structure it forms an important lymph-space.
[The muscle-fibre of the cutis is very unevenly distributed. It is found somewhat freely in the region of the back, the dorsal surface of the head, and the neck; less freely on the dorsal surfaces of the extremities, very sparsely on the abdomen, breast, and ventral surfaces of the extremities; in the feet it appears to be absent (Eberth).
The pigment of the skin. As already mentioned pigmented cells occur in the epidermis, but by far the greater quantity of the cutaneous pigment is found in the cutis, and more especially in the loose layer of branched cells, which form the boundary between epidermis and true cutis. These cells, known as chromatophore-cells, have been carefully investigated by Harless. They play an extremely important part in bringing about the well-known changes in the colouring of the skin. Bimmerman has proved that these cells are influenced by stimulation of the nerves supplying the region in which they occur. Ehrmann has been able to trace a direct connection between the nerve-fibres and the pigment-cells (Fig. 231 I, II). In such cases the pigment was sometimes sharply marked off, at others was gradually lost along the nerve.]
c. The papillae and ridges of the skin are classified by Leydig as follows: (1) Small ridges, (2) larger ridges, (3) papillae with touch-bodies, (4) papillae without touch-bodies, (5) papillae with the ducts of glands, (6) capillaries in the form of papillae, (7) small elevations, including the lateral organs.
[All these structures are derived from the cutis, the epithelium above simply repeating the elevation, and being itself either not increased in thickness or only to a slight degree. (For further description of some of these papillae, see Organs of Tactile Sensation, p. 380.)
Peculiar wart-like papillae are found on the articulations of the first phalanges with the metatarsus, on the same articulations and on the articulations between the first and second phalanges of the third and fifth toes, and on the articulations between the first, second, and third phalanges of the fourth toe. These papillae are connected with the long flexor tendons by fine tendinous bands (Klein).
Temporary papillae are developed in the females of Rana temporaria during the breeding season; according to O. Huber (l. c.), who has described them, their distribution and structure is as follows:—
During the breeding season the skin of the sides of the trunk of female specimens of Rana temporaria is rough; the small elevations, to which this roughness is due, are each about the size of the head of a pin, and have the appearance of small, rounded, white or pale rose-coloured, warty growths. The papillae reach their greatest development and are most numerous on the sides of the trunk (Fig. 232 I); they may be traced on to the tympanic membrane, where they are arranged in a ring, and in some cases under the eyes to the sides of the snout.
Anteriorly the papillae are absent from the median portion of the dorsal surface of the trunk, but posteriorly, behind a line joining the anterior extremities of the two iliac bones, the papillae cover the whole surface.
The papillae are large and very numerous around the arms, and pass by gradual, transitionary stages into the ordinary papillae of the region. Further they may be traced backwards along the anterior or extensor surface of the thigh, the upper surface of the leg, foot, and fifth toe. Occasionally a few papillae were found on flexor surfaces of the third and fourth metatarsus, and very rarely were any discovered on the fore-limbs. They occur in no other batrachian of Germany (Huber).
Fig. 232.
I. Diagram to show the distribution of the temporary papillae in Rana temporaria; after Huber.
II, III. Vertical sections through a temporary papilla; after Huber.
Each papilla (Fig. 232 II and III) consists of a hemispherical elevation of the cutis, and is covered either by a layer of normal epidermis or in some very rare cases the epidermis was slightly thickened. The papilla consists of a firm connective-tissue stroma, and is from 0.2–0.4 mm. in height, 0.4–0.5 mm. broad (after hardening in alcohol). Such glands as exist in the papilla usually open laterally (Fig. 232 III), and the blood-vessels are numerous.
The epidermis covering the papilla is well supplied with nerve-fibres, derived from large ganglion-cells in the papilla (Fig. 232 III). The branched pigment-cells, so numerous in the rest of the skin, are absent from the upper parts of the papillae (Fig. 232 II); the pigment-cells may cease either abruptly or gradually.
The rose-colour of the papillae is due to the presence of a golden red pigment. After the breeding season the papillae are gradually lost, and the pigment-cells wander into the unoccupied parts.
Huber compares the arrangement of the nerves to that of the ‘touch-spots’ of Merkel.
d. The glands of the skin are of two kinds, serous and mucous; they have been fully described by Engelmann and Openchowski.
(1) The serous glands (Körnerdrüsen, Engelmann) are large, averaging in diameter 0.2–0.4 mm., but others vary in size from 0.13–0.8 mm. in diameter. They are found chiefly on the dorsum of the trunk and arranged in groups, which vary in dimensions from 3–20 mm. in length and 2–4 mm. in breadth. They may be found in any part between the ear-region and the anus, on the dorsal surface of the thigh, especially towards the outer and inner borders. They are found very sparsely distributed on the ventral surface of the trunk and on other parts of the extremities, though as a rule a few are found on the dorsal surface of the feet and near the phalanges. They are wanting in the nictitating membrane.
The glands have a rounded form, with the ducts placed at their superficial poles. The glands possess three coats; externally is a coat of connective-tissue, the fibres of which are arranged closely together, and cross one another at acute angles: these fibres are continuous with the connective-tissue of the surrounding cutis. Inside this coat is a thick layer of muscle-fibre (0.005–0.015 mm.); the muscle-cells are arranged meridionally, with the one pole towards the superficial surface of the gland, the other pole at the deep surface (Hensche). Many of the fibres, however, extend only one-fourth or one-third of this distance.
The innermost coat is a simple layer of epithelial cells, which rests directly on the muscle-layer; no basement-membrane or space of any kind having been discovered between the two layers. When examined in the recent state these cells are conical or cylindrical in shape, and have the appearance of goblet-cells: they possess a delicate but distinct cell-wall, but are open towards the lumen of the gland. One or sometimes two rounded nuclei are found towards the base of the gland, and surrounded by a small amount of granular protoplasm; the rest of the cell is occupied by a mass of small, rounded, highly refractive granules of about 0.002–0.01 mm. in diameter. These granules are also to be found in the protoplasm of the cell; they contain a substance capable of swelling freely, and present many points of resemblance to the granules found in the cells of the oviducts.
These glands represent the poison-glands or lateral glands of toads, and the ear-glands or parotids of Caecilia (Leydig). According to Leroux81, Gratiolet and Cloëz82, the reaction of these glands in toads varies; according to du Bois-Reymond83 the reaction is, in frogs, acid; Engelmann84 found the reaction of these glands, in frogs, to be neutral, frequently, however, with a tendency to either acidity or alkalinity.
These glands correspond with the ‘large glands’ of Hensche, the ‘contractile glands’ of Stieda, the ‘large, dark glands’ of Eberth, and the ‘large contractile glands’ of Leydig’s earlier publications.
(2) The mucous glands are, as a rule, smaller, more numerous, and more evenly distributed than the serous glands. Usually they lie in such close proximity as almost to touch one another; according to Engelmann an average number of sixty is found in one square millimeter; on the abdomen 62–68; flexor surfaces of the posterior extremities in some places 76, but in others only 30–40; on the webs only 2–6 to the square millimeter. On the nictitating membrane they are arranged in two or three parallel rows, are large, and closely applied to each other. They are wanting on the deeper surface of the nictitating membrane.
The glands are generally rounded in form and have a short neck (Fig. 229). Glands in the recent condition, and not fully contracted, have a diameter of 0.06–0.21 mm., the majority measure 0.12–0.16 mm. in diameter. The upper pole of the gland is usually placed directly under the boundary between epidermis and cutis, and, as a rule, about 0.06–0.1 mm. beneath the superficial surface of the skin. The duct usually passes directly to the surface, in a straight line.
The structure of the glands corresponds, in general, with that of the serous glands. Externally is a coat of connective-tissue, which is, however, more firmly attached to the muscular coat than in the case of the serous glands, and may easily be mistaken for a structureless basement-membrane85 (Eberth). The muscular coat is very thin, and consists of 16–20 flat, spindle-shaped fibres; extending from the upper to the lower pole as in the serous glands. At a short distance (0.015–0.022 mm.) from the upper or superficial pole each fibre has an ellipsoid thickening, composed of granular protoplasm, and containing a clear oval nucleus.
The inner coat is composed of a single layer of epithelium, placed directly on the muscular coat. The epithelium has a thickness of about 0.01–0.02 mm.; it is, however, usually thicker towards the base than towards the duct. The boundaries between the cells are very indistinct, and so give the epithelium the appearance of a continuous layer of protoplasm. The glandular epithelium is not directly continuous with the epithelium of the duct.
The shape of the cells depends upon their physiological condition. In a state of semi-contraction of the gland, the cells are cubical, and their free borders pushed into the lumen in a slight convexity; when the gland is fully distended the cells are two or three times as broad as deep; in a fully contracted gland the cells are conical or cylindrical. In the first-mentioned condition their diameter varies from 0.006–0.018 mm. The larger glands are estimated to possess about 150 cells, the smaller only thirty to forty (Engelmann).
The cells appear to have no cell-wall, certainly they possess none on their free surfaces. Their protoplasm distends very freely on the addition of water. As a rule some cells are much more granular than others of the same gland; each cell has a rounded oval, clear nucleus, and vacuoles are frequently met with.
The lumen of the gland contains a colourless, transparent, watery fluid, which contains mucous material.
The ducts of the glands are thick-walled cylinders, circular in section. Their diameter varies from 0.02–0.05 mm., their length is equal to the depth of the superimposed epidermis. The wall of the duct is lined with two or three concentric layers of highly refractive, nucleated cells. The cells are arranged parallel to the length of the duct: in transverse section they are sickle-shaped or semilunar. In the upper third or fourth of the duct the inner wall is lined with a cuticle, very resistant to the action of acids or alkalies: it is thrown off with the skin in the process of casting. In the horny layer of the epidermis this cuticle is partially closed by a stoma-cell (Eberth), (Fig. 230), which possesses a tri-radiate opening.
The glands correspond with the ‘non-contractile glands’ of Hensche, the ‘dark and bright glands’ of Stieda, the ‘glandule mezzane, piccole e piccolissime (ad epitelio pavimentoso)’ of Ciaccio, and the ‘small, dark, and medium-sized, bright glands’ of Eberth.
Fig. 233.
Vertical section through the epidermis of the supplemental toe of Rana esculenta; after E. Schultze. Magnified 400 times.
e. The peculiarities of the skin of the supplemental toe and webs. The supplemental toe presents, in the male, a rounded, oval swelling (Fig. 234 I, D), with the end of the digit projecting from the distal end.
Normally this swelling is usually of a grey colour; during the breeding season, however, the swelling enlarges, and becomes of a black or deep brown colour (Leydig).
The epidermis covering this swelling (Fig. 233) is much thickened and produced into projecting papillae. The cutis also shows a marked thickening and an increase in vascularity; the glands are of the kind described as mucous glands (Engelmann). They present, however, an immense increase in size, and at the same time are so closely applied to one another as to form the main part of the enlargement (see also Organs of Tactile Sensation).
In the webs the various layers of the corium are more or less fused together, and cannot be distinguished from each other; the glands are, as a rule, smaller than on the general surface of the skin.
Fig. 234.
| Fore-foot of a male frog. |
Vertical section through the swelling on the supplemental toe of a male frog. | ||||||||||||||||
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f. The blood-vessels and lymphatics of the skin.
(1) The blood-vessels of the skin are arranged in two networks; the one is situated immediately beneath the epidermis, and consists of fine capillaries, forming a fine-meshed network, which enclose the ducts of the glands. The meshes vary considerably in size and shape (Hyrtl and Langer). The second network of vessels is situated under the cutis; the vessels forming it are small, and the meshes formed are large. From this network vertical branches pass to form the superficial network; these branches usually course along the strands of vertical fibres already described, and break up to form capillaries only when they reach the deeper surface of the epidermis (Langer), (Fig. 235 I).
Fig. 235.
The blood-vessels and lymphatics of the skin; after Langer. Arteries striped, lymphatics shaded.
| I. | Vertical section through skin of thigh. |
| II. | Horizontal view of arteries and lymphatics of the skin. |
| III. | Peculiar tortuous arteries of the lamina inguinalis. |
| IV. | Arteries and lymphatics of the web. |
In some situations, however, the vertical branches divide earlier; this is especially the case in parts where the glands are very closely packed, as in the nictitating membrane, the upper lip, and the swelling of the supplemental toe (in the male). In these parts the division takes place at the deeper surface of the glands (Langer).
A peculiar arrangement of the cutaneous blood-vessels is found in the lamina inguinalis, where a chain of reddish points can be made out with the naked eye; examined more carefully (Fig. 235 III) each reddish point is found to be a complex loop of capillaries. They extend from the skin to the hinder lymph-hearts, where they form a close network which partially invests the lymph-hearts (Langer).
(2) The lymphatics of the skin (see also p. 252), like the arteries, reach the deeper surface of the epidermis by coursing along the bands of vertical fibres; they then form a network of capillaries with rounded meshes (Fig. 235 I, II, IV), and lying immediately under the epidermis, and a network which surrounds the various glands. The lymphatic-capillaries are as a rule larger than the blood-capillaries. The lymphatic network lies beneath the blood network, and the vessels of the two systems branch independently (Langer).
In those parts where the glands are placed closely together this arrangement is modified, and corresponds with the modifications found in the blood-capillaries in these regions. The vertical branches divide so as to form a network below the glands, and from this vertical branches pass in the same direction between the glands to form a secondary network on the superficial surface of the glands (Langer).
g. The nerves of the skin. The cutis is very richly supplied with both medullated and non-medullated nerves. In the subcutaneous tissue the nerves destined for the skin branch freely to form numerous fine twigs, which, without actually inosculating, form a fine meshwork. From the larger trunks numerous vertical branches pass vertically to form a secondary network, from which both coarse and very fine twigs pass to encircle the glands. The fine fibres which supply the glands are non-medullated and possess oval nuclei; their diameter varies from 0·001–0·002 mm. The number of nerve-fibres is not much smaller than that of the muscle-fibres (Engelmann). (See also Organs of Tactile Sensation.)]
The organs of tactile sensation are the nerve-plexus of the epidermis, the touch-spots of Merkel, the lateral sense-organs, and the touch-corpuscles of the tendons.
a. [The nerve-plexus of the epithelium is very incomplete as compared with the plexus found in the epithelium of the cornea; here and there a few non-medullated fibres have been traced a short distance between the epithelial cells. No such mode of nerve-terminations in the epithelium as is described by Eberth and Macullum in the tadpole can be found in the adult.]
b. The touch-spots. What knowledge we possess of these organs is due to the labours of Leydig, Ciaccio, Eberth, and more particularly those of Merkel.
At the base of each papilla, which contains such a touch-organ, is found a number of colourless, flattened cells, arranged either in a single layer, or heaped up and connected with very fine nerve-fibres. The flat surface of the cells is parallel to the surface of the body, and they do not form a separate or circumscribed body, they are therefore better spoken of as touch-spots than as true touch-bodies (Merkel). The larger organs are found in the papillae, but smaller ones may be found on any part of the skin.
Fig. 236.
| I. | Lateral sense-organ of tadpole of frog. Half-schematic. | ||||||||
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| II. | Touch-corpuscle from the sheath of a digital tendon of the frog; after Löwe. Schieck, Oc. O., Obj. 9. |
These organs are best seen in the prominence or swelling upon the supplemental toe during the breeding season; they are numerous on the dorsal surface of the trunk, but occur most frequently on the under surface of the hinder feet (Planta pedis). After the breeding season the organs of the thumb undergo a retrogressive change, which results in a network of spindle-shaped and branched cells with fibres (Wiedersheim).
c. The lateral sense-organs attain their highest development in the tadpole; in the adult the organs have undergone a retrogressive change, the result of which is that the organs are diminished in size; the whole organ sinks by the formation of a tube, which is then closed by a mucous secretion, consequently the organ is functionless: with this loss in function and change in position of the organs a corresponding diminution of the ramus lateralis nervi vagi occurs (Merkel).
Each lateral organ (Fig. 236 I) consists of a slightly elevated papilla, with the centre depressed, and in each may be distinguished a central and a peripheral zone (c, p). The central zone consists of a group of pear-shaped cells, with the narrower ends directed towards the free surface; each is connected below with a nerve-fibril, while above it bears a short, stiff cilium. The cilia are enclosed in a delicate hyaline tube (R), which is open superficially, closed by the papilla below; consequently the cilia are in direct contact with the surrounding medium. The peripheral zone is a layer of pale, flattened, cylindrical cells (p); they possess, at their upper borders, a perforated membrana limitans (Ml), through which the cilia pass. These cells serve as a support to the central cells, and are themselves surrounded by ordinary epithelial cells.
The distribution and arrangement of the lateral organs is alike in fish and larval amphibia. They are most numerous on the head, where they surround the eye, and are continued forwards to the snout and on to the lower jaw. All these organs on the head are in connection with the trigeminal nerve. A line of these organs passes from the hinder part of the circumference of the eye along the gill-cover to the neck, where the lines of opposite sides are usually, though not constantly, joined by a transverse line of the same organs; thence the lines are continued along the trunk to the tip of the tail. In fish there exists, as a rule, only one pair of such longitudinal lines (Linea lateralis); in anura-larvae, proteus, and in all salamanders there are three pairs: of these one pair lies near the vertebral column, the second at the junction of the flank with the abdomen, and the third corresponds to the lateral line in fish (Malbranc).
At an early developmental period each metamere possesses one pair of such lateral organs; in later life, however, this simple arrangement is lost, and each segment usually possesses a group of organs.
The organs of the trunk and tail are supplied by branches of the Ramus lateralis nervi vagi. Both in fish and amphibia a reparative process, by means of fission, occurs; as a consequence the organs are found in various stages of development.
d. [The touch-corpuscles of Golgi and Löwe (Fig. 236 II) are found on the joints of the digits. They are surrounded by connective-tissue and are spindle-shaped; the two ends of the spindle are glassy in appearance, the middle portion is fibrous. In the thickest part of the corpuscle is a zone of nuclear bodies.
They are regarded as touch-bodies (Golgi and Löwe). Golgi (l. c.) describes two kinds of touch-organs in connection with muscle and tendon; one class correspond with those just described, he names them ‘nervous muscle-tendon organs.’ The second class he compares with the touch-bodies found in the conjunctiva.]
The organs of taste are not confined to the surface of the tongue, but are also found on the mucous membrane of the roof of the mouth, especially in the neighbourhood of the vomerine teeth; on both roof and floor of the mouth these organs can be traced to the commencement of the oesophagus.
[The mucous membrane of the tongue possesses two kinds of papillae and numerous glands.
a. The filiform papillae are the more numerous; they are conical or thread-like in form, and consist of connective-tissue, with a few striated muscle-fibres; they include blood-capillaries, but no nerves have been traced into them (Leydig). They are covered with ciliated epithelium and goblet-cells.
b. The fungiform papillae are much larger, and are paler than the filiform papillae. The free ends of these papillae are broader than the bases, and when the epithelium is removed the free ends present a concavity. From the base to the border of the free surface each papilla is covered with the usual oval, ciliated epithelium, but at this border an abrupt transition takes place. The epithelium covering the end of the papillae is of three kinds: goblet-cells, cylindrical cells, and forked cells; the three forms of cells are quite characteristic, and no intermediate transitionary forms are met with.
(1) The goblet-cells (Fig. 237 I) are arranged vertically to the free surface of the papilla; they are from 0.020–0.024 mm. in length, and from 0.01–0.02 mm. in diameter. In the lower third of the cell is found a nucleus of about 0.008 mm. in diameter, and this encloses a nucleolus of about 0.001 mm. in diameter. Close underneath the nucleus the cell is contracted to form an irregular process or foot. The contents of the cells are a very finely granular, transparent protoplasm. These cells form the outermost part of the epithelium, covering the end of the papilla; all the cells belonging to the same papilla are of the same size. In consequence of the mutual pressure which they exert on each other they present, in transverse section, six-sided outlines (Fig. 237, 2). The nuclei of adjacent cells are placed at almost exactly the same level. By the action of reagents on these cells very remarkable forms are obtained, in consequence of the protoplasm swelling and being forced upwards (Fig. 237, 5).
Fig. 237.
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(2) The cylindrical cells (Fig. 237 I) have rounded free extremities, which reach to the general surface of the epithelium; the remainder of the cell is cylindrical except towards its lower extremity, where it is slightly dilated and encloses an oval nucleus: the protoplasm of the cells is very finely granular. These cells rest on the connective-tissue of the papilla, and are in close juxtaposition, so that several hundreds of them are found on one papilla.
The spaces between the upper parts of the cylindrical cells are occupied by the goblet-cells and by the forked cells.
(3) The third form of cell found on the fungiform papillae are named forked cells (Fig. 237 I) by Engelmann. The body of the cells has an ellipsoidal form, is from 0.006–0.008 mm. in its longer diameter, and 0.003–0.004 mm. in the shorter diameter. Processes arise from both poles. The peripheral processes arise by a short common stem, which then divides into two or rarely three branches; the whole process is from 0.004–0.008 mm. in length, and is always just sufficiently long to reach the general surface of the epithelium: when the peripheral process is long the central is usually shorter, and vice versâ.
The central processes arise by a stem from 0.001–0.002 mm. in thickness, which usually divides dichotomously. The length of this process may be as much as 0.025 mm. or almost nil.
The forked cells are about twice as numerous as the goblet-cells, they occupy the spaces between the cylindrical cells and the goblet-cells. The branched central processes form a network on the connective-tissue of the papilla, which is, at this point, perforated by a rich plexus of fine non-medullated nerve-fibrils. It is not decided whether the processes of various cells inosculate. Engelmann regards these forked cells as the taste-cells and as the sensory nerve-endings.
The bodies of the fungiform papillae contain blood-vessels and nerves, the latter enter as medullated nerves, but lose the medulla, somewhat abruptly towards the upper end of the papilla, where they form a sort of nerve-cushion (Nervenkissen, Engelmann).]
Taste-organs are also found on the roof and other parts of the mouth, and present many points of resemblance to the lateral sense-organs. Like them they present a peripheral zone and a central zone; the cells of the latter, however, are not longer than the peripheral cells, and possess no cilia (J. van der Hoeven, Merkel).
The anterior nares or nostrils are two small openings, placed directly in front of the anterior angle of the eye; the distance between the eye and the corresponding nostril being, in the case of adult animals, about five or six millimeters. The anterior nares are surrounded by rims, which are contracted below, and so form very short, tentacular-like prominences.
The posterior nares (choanae) are about four millimeters from the anterior nares, consequently the long axis of the nose is, approximately, of this length. (For the external muscles of the nose, see p. 59.)
The boundaries of the nasal cavities are as follows:—the roof of each cavity is formed by the dorsal plate of the sphenethmoid, the nasal bone, and the premaxillary bone; the floor is formed by the vomer and the palatine bone, the inner wall by the vertical septum of the sphenethmoid, the outer wall by the premaxillary and maxillary bones, the anterior wall by the premaxillary bone, and the posterior wall by the sphenethmoid. The cartilaginous portion of the nasal skeleton (see also p. 27) projects into the general cavity and subdivides it into various sinuses, which have been the subject of careful investigation by Born and Wiedersheim.
a. The nasal cavities are best examined by means of serial transverse sections; in a section through the anterior nares (Figs. 238, 239) there will be seen three sinuses on either side:—
(1) The superior sinus (on) is large, rounded, and placed against the cartilaginous septum; the sinus is lined with olfactory epithelium. Posteriorly the sinus extends beyond the posterior nares, and is bounded by the anterior surface of the sphenethmoid. In front it ends in a rounded concavity, likewise lined with olfactory mucous membrane. Opposite the anterior nares the cavity is partially subdivided, by a longitudinal process on the floor, into two cavities; the anterior nares open into the external chambers, and therefore not directly into the main cavity of the superior sinus. The superior sinus communicates with the inferior sinus by a narrow, almost vertical, slit.
(2) The inferior sinus (un) is narrow and flattened from above downwards; it is placed against the maxillary bone, and represents the maxillary sinus of the frog. Externally it possesses a descending arm.
The inferior sinus is continued backwards and opens on its inner side into the posterior nares, or rather into the mouth. The descending arm extends only a short distance forwards, to terminate near the point where the superior and inferior sinuses communicate by only a narrow opening. This sinus is on the whole broader in front than behind, and has a general direction from without, inwards and forwards underneath the superior sinus.
Fig. 238.
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Fig. 239. |
Frontal sections through the nose of two tadpoles; after G. Born.
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(3) The lateral sinus (sn) is situated in the partition between the superior and inferior sinuses, or rather between the external chamber of the upper sinus and the inferior sinus. The lateral sinus is triangular in form, being narrow in front and broad behind; at its inner posterior angle it opens on the free border of the horizontal partition between the superior and inferior sinuses; this opening is, however, continued along its roof, so that the lateral sinus opens also into the superior sinus.
b. The nasal cartilages. The two nasal cavities are completely separated by the cartilaginous septum (Figs. 238 and 239 S), and are for the most part lined by cartilage. The anterior end is formed of concave cartilages, while the posterior, being situated in the sphenethmoid, is usually more or less ossified in the adult. The posterior wall has two openings: a larger near the septum for the olfactory nerve, and a smaller, more externally, for the nasal branch of the trigeminal nerve.
The anterior wall is more complex than the posterior, being thicker in the middle than at the sides, and possessing three blind sacs for the three sinuses. From the anterior wall two processes project backwards between the sacs and enclose them more or less completely.
Of the three sacs or cavities only the lower is completely surrounded by cartilage. The upper sac is in part bounded by a shell-shaped, concave cartilage (Concha narium, of various authors; Os lachrymale, Born), which covers it anteriorly and externally, and is attached by a small base to the roof of the lower sac (Fig. 239, l), its upper border bounding the anterior naris (Fig. 15). The upper and external parts of the upper sac have no cartilage. The cartilages of the outer wall of the lower, blind sac extend backwards to the point where the maxillary sinus commences to descend: the roof, however, is prolonged further backwards by two small cartilaginous processes; the inner is short, the outer forms the roof of the descending arm of the sinus, and joins a cartilaginous process, which commences at the anterior portion of the roof of the nasal cavity, passes backwards and downwards to a broad plate, and forms an incomplete outer wall to the nasal cavity. There are also three cartilaginous processes—α. One arising from the outer side of the floor at the level of its junction with the anterior wall; it is a flattened process and passes outwards, and bifurcates at its end to meet the premaxillary and maxillary bones (this is the Oberkieferfortsatz of Ecker), (p. 28, Fig. 14 n″). β. A delicate process on either side, described by Wiedersheim (see p. 280). γ. The third pair of processes are fully described for the first time by Born; each arises at the lower border of the corresponding Concha narium, passes forwards and downwards under the ascending process of the premaxillary to the point where process β is attached to this bone. The processes are flat and approach one another obliquely; in older animals they are united at the point of contact.
At the junction of the anterior wall, the septum, and the floor is a large aperture in the cartilaginous capsule, through which passes the chief nasal branch of the trigeminus, which supplies numerous branches to the intermaxillary gland.
c. The glands of the nasal region are Bowman’s glands, the intermaxillary glands (Wiedersheim), the lower nasal gland (Born), the upper nasal gland (Born), the pharyngeal gland (Born), and the lachrymal duct.
(1) The intermaxillary gland (see p. 280).
(2) The lower nasal gland is placed along the septum and extends as far back as the posterior nares, opening by its ducts (Figs. 238 and 239 gni) into the inferior blind sac of the nasal cavity. In histological structure it corresponds with the intermaxillary glands, except that the glandular tubes are somewhat smaller, and the glandular epithelium stains somewhat less easily with carmine (Born).
(3) The upper nasal gland occupies the space between the Concha narium (Os lachrymale, Born) and the neighbouring cartilages, it also surrounds the anterior naris and the opening of the lachrymal canal (Figs. 238 and 239 gns). The numerous ducts open on the mucous membrane covering the oblique cartilage (above described, as passing from in front, downwards and backwards, and forming an incomplete outer wall of the cavity), and its hinder prolongation.
(4) The pharyngeal gland is placed transversely behind the posterior nares, and surrounds the vomerine teeth. A portion of the ducts open into the posterior nares, the remainder on the mucous membrane of the pharyngo-oral cavity at two symmetrically placed points (Fig. 238 R).
(5) The lachrymal duct (see p. 428) opens into the nasal cavity at the point where the lateral sinus opens into the external chamber of the superior nasal sinus.