I. THE LARYNX.

The larynx (Fig. 202) is a short wide tube placed between the posterior cornua of the hyoid, to which it is attached by connective-tissue. The long axis of the tube lies in the median line and almost horizontally, but the posterior end is on a slightly lower level than the anterior, when the animal is in the natural sitting position (Fig. 202). The anterior end of the larynx opens into the mouth by a longitudinal slit (Fig. 179 L), and is placed in a slight depression caused by the folding of the mucous membrane; the posterior end communicates with the cavities of the lungs.

The larynx is lined with mucous membrane, which is continuous, in front with that of the mouth, behind with that of the lungs. The organ has a skeleton of cartilage, and possesses special muscles, by which the supply of air to the lungs, and the voice can be regulated.

a. The cartilages of the larynx.

The cartilages of the larynx are five in number, of which four are paired and one is single.

(I) The cricoid cartilage (Figs. 203, 204) is an oval ring of cartilage with various processes. The ring-like portion of the cartilage is placed in a plane which is almost vertical, but which is directed slightly upwards anteriorly, and slightly downwards posteriorly (the animal being in the usual sitting position).

The sides of the ring are slightly curved inwards on the anterior surface, and possessed of irregular enlargements (C.l.¹-C.l.⁴), the space enclosed by this portion of the cartilage is occupied by a membrane (M), which forms the floor of the body of the larynx.

From each side of the body of the cartilage, a process (C.l.²) is given off, which curves backwards and inwards to join its fellow of the opposite side, the two forming a blunt spinous process (Sp), which projects backwards, and is intimately attached to the oesophagus. Between these processes and the lower portion of the body of the cricoid cartilage are the apertures of the roots of the lungs (Fig. 203), which, by their attachments to these cartilages, are kept open.

(2) The arytenoid cartilages (Fig. 203 I, II, Ca, Ca¹) are a pair of cartilages placed in front of the cricoid cartilage, one on each side. Each cartilage is semilunar in shape, concave internally, and convex externally. The cartilages are placed almost vertically, with their posterior borders or bases parallel to the body of the cricoid cartilage. The superior borders (Fig. 204) are directed upwards and forwards, the inferior downwards and forwards. The superior and inferior borders are separated by a semicircular notch, bounded by two sharp apices. The superior and inferior angles of the two cartilages are close together, and movably attached to each other by connective-tissue.

These cartilages vary very greatly in the two sexes. In the males they are thick, strong, and large; in the female the cartilages are very thin, more hollowed and much smaller.

(3) [The pre-arytenoid cartilages (Fig. 206 I, P) are two small elongated cartilages placed in the semicircular notch between the superior and inferior borders of the arytenoid cartilages. They are subject to much variation in size, sometimes being merely a very slender rod, at others a moderately thick oval mass. In female specimens they appear to be, at times, absent, or to unite with the arytenoid cartilages, as in these cases a third very small apex is found on each arytenoid cartilage; but it is always much smaller than the two neighbouring apices.]

The muscles of the larynx appear in the following order, when dissected from the mouth:‍—

b. The attachments of the cartilages to each other.

The cartilages do not articulate directly with each other, but are connected by connective-tissue only; there are, therefore, no synovial sacs.

c. The muscles of the larynx‍75.

(1) The M. dilatator aditus laryngis (Henle), (Fig. 205 D.l.) arises on either side from the hinder end of the larger posterior cornu of the hyoid: the fibres diverge slightly to be inserted into the middle portion of the outer surface of the arytenoid cartilage; a smaller bundle of fibres is attached to the deeper-lying constrictor muscle and to the cricoid cartilage.

(2) The M. constrictor aditus laryngis (Henle), (Fig. 205 C.a.l.), arises on either side from the hinder half of the dorsal surface of the posterior cornu of the hyoid. The two muscles enclose the larynx, and are inserted into a median tendinous raphe on the under surface of the larynx (Jt). The raphe is connected with the skeleton of the larynx by connective-tissue.

(3) The M. hyo-ary­te­noi­deus anterior (Fig. 205 C.o.l.) arises on each side from the inner border of the anterior end of the cornu of the hyoid; the muscle lies close to the anterior border of the arytenoid cartilage, and is inserted into a fibrous lamella on the dorsal surface of the larynx. The M. petrohyoideus tertius is also partially inserted into this lamella.

(4) The M. petrohyoideus tertius (Fig. 205 Ph⁴), (see also p. 66). The greater part of this muscle is inserted into the end of the posterior cornu of the hyoid (HH); a smaller portion (S, S²) is prolonged to be inserted into the cricoid cartilage and into the fibrous lamella into which the MM. hyo-arytenoidei anteriores are inserted.

(5) The M. hyo-arytenoideus posterior (Fig. 205 C.o.l.¹) arises on each side from the superior angle of the corresponding arytenoid cartilage, under cover of the tendon of the M. petrohyoideus tertius. The muscle is inserted into the inferior angle of the arytenoid cartilage.

d. The interior of the larynx (Figs. 202, 206, 207). The cavity of the larynx is constricted at two points: anteriorly it is constricted by the true vocal cords (Figs. 206 V, 207 SB), posteriorly by the false vocal cords. The whole cavity is lined with mucous membrane, which is continuous with that of the mouth anteriorly, with that of the lungs posteriorly.

(1) The true vocal cords are two vertical flat bands of connective-tissue, attached above to the superior angles of the arytenoid cartilages, below to their inferior angles; their anterior borders are thin and free; near their posterior borders they are attached by mucous membrane to the internal surfaces of the arytenoid cartilages. The anterior and posterior borders are not parallel but are each concave (Fig. 202 V).

The anterior border is thin, the posterior thick and rounded.

Seen from in front (Fig. 206), the opening between the cords (Rima glottidis) is slightly wider at each end than in the middle. The ends of the concave posterior border are prolonged backwards and enclosed in a fold of mucous membrane. Part of the tissue enclosed is unstriated muscular fibre, which may be traced to the cricoid cartilage.

(2) [The false vocal cords (Figs. 202 and 206 F) are simply folds of mucous membrane, which extend vertically on each side of the larynx behind the true vocal cords; they do not extend so far towards the median plane as do the true vocal cords.]

(3) [The ventricles of the larynx (Figs. 202 and 206) are two on each side. The anterior ventricles are between the true vocal cords and the arytenoid cartilages; they are open anteriorly, and closed by mucous membrane posteriorly.

The posterior ventricles open towards the median plane, each presenting an oval opening (Fig. 202), which widens outwards into a large cavity (Fig. 206 II). The cavity is bounded in front by the base of the true vocal cord, and the mucous membrane attached to it; posteriorly by the false vocal cord, and externally by the cricoid cartilage and the connective-tissue capsule of the larynx.]

(4) [The mucous membrane of the larynx varies in structure in various parts of the organ. From the anterior opening of the larynx to the posterior borders of the vocal cords it is lined with stratified epithelium, which is firmly attached to the underlying structures by a small amount of sub-epithelial tissue. This is especially well marked on the vocal cords themselves.

Behind the vocal cords the mucous membrane is much more loosely attached to the surrounding structures by an extremely vascular areolar tissue. The epithelium is arranged in a single layer of columnar cells, among which are numerous goblet-cells. In the more external parts of the posterior ventricles, the mucous membrane is thrown into deep folds and so forms polygonal acini. In the median line of the floor of the larynx and behind the false vocal cords is a vertical fold of mucous membrane, which increases in height and breadth as it proceeds backwards towards the roots of the lungs. The mucous membrane behind the true vocal cords is extremely vascular, in the most posterior portion of the larynx the blood-vessels form a capillary network exactly like that of the lungs.]

e. [The epiglottis (Fig. 206 E) is a small bilobed fold of mucous membrane placed on the floor of the mouth in the median plane and immediately in front of the aperture to the larynx. Between it and the mucous membrane covering the arytenoid cartilages are a number of large mucous glands (G). The epiglottis does not contain cartilage; it is, however, constant in its appearance and sharply marked off from the surrounding mucous membrane.]

II. THE LUNGS.

a. General description. The lungs are two large thin-walled sacs (Figs. 185 and 204 Lg and Lg¹). The roots of the lungs are contracted at their origin from the larynx and then expand to form two ellipsoid sacs, which terminate posteriorly in bluntly-pointed ends. With the exception of their roots they lie entirely free in the pleuro-peritoneal cavity, and are covered by the pleuro-peritoneal membrane. In the recent state they have a bright red colour due to the large supply of blood-vessels.

b. Minute structure.

(I) The muscular tissue of the lungs is for the most part arranged in large bands, which form a coarse network on the deeper surface of the organ; when seen in section‍76 (Pl. II, Fig. 208 A, B) these bands are found to be composed of well-developed involuntary muscular fibres. Between the larger bands are smaller bands having a similar arrangement. From these networks of muscular bands finer processes of muscular tissue pass peripherally towards the surface of the organ, and are attached to the thin and incomplete muscular layer found in the wall of the lung (C).

(2) The connective-tissue of the lungs is present in only small quantity, but is still sufficient to fill in the spaces between the various muscular bands and the surface of the lung, and to invest the whole of that surface. There is thus formed a series of pits, the mouths of which open into the general cavity of the lung, while their bases are at the surface. Through this connective-tissue course the blood-vessels, nerves, and lymphatics. It contains numerous yellow elastic fibres.

(3) The blood-vessels of the lungs. The pulmonary artery courses along the outer surface of the lung to the apex, giving off, at right angles, lateral branches in the whole of its course; these show a tendency to be alternately larger and smaller. The lateral branches divide and form a rich capillary network (T. Hoffmann).

The capillary network has very small meshes; the diameter of a given mesh being frequently less than that of the capillary bounding it. The meshes are rounded or polygonal in shape.

The pulmonary vein arises by lateral branches from this capillary network; the branches join, at right angles, the main vein, which courses from the apex of the lung along its inner surface to the root of the organ.

(4) The epithelium of the lungs. Externally the lungs are covered with a layer of endothelium derived from the peritoneum. Internally the surface is covered with an epithelium which varies considerably in different positions.

On the free borders of the muscular trabeculae forming the borders of the alveoli is a short columnar ciliated epithelium (Fig. 208) such epithelium is also found in the root of the lung; it contains goblet-cells.

The alveoli, for the most part, are lined with a single layer of tesselated epithelium; the cells are polygonal in outline, with finely granular contents and a distinct nucleus: the average diameter of the cells is from 0.0074 to 0.0108 mm., that of the nucleus 0.0054 mm., that of the nucleolus 0.0009 mm.; four to eight of such cells occupy the space enclosed by one mesh of the capillary network (Eberth).

The epithelium rests on a structureless basement membrane, which is continuous over the whole inner surface of the lung, whereas the epithelium does not pass over the capillaries, and is therefore only found in isolated patches in the areas enclosed by the capillaries (Eberth).

In various isolated spots, small groups of short columnar or goblet-cells are found in the tesselated epithelium (Eberth, Hoffmann).

(5) [The lymphatics of the lungs have been described by T. Hoffmann; they form a network of vessels surrounding the larger blood-vessels: from this branches are given off, which form a network of fine canals through the whole of the lung; part of this secondary network accompanies the blood-capillaries, but other portions run a separate course. They communicate with the pleuro-peritoneal cavity.

(6) The pigment-cells are very numerous, branched, and large; they accompany the lymphatics, and not the blood-vessels (T. Hoffmann).

(7) The nerves of the lungs (p. 172) course along the larger blood-vessels, under the serous coat; the fibres are chiefly medullated fibres (Egorow, Kandarazki). Non-medullated branches, which form a plexus in each alveolus, are given off. The branches have small triangular enlargements (ganglia), where they unite. The nerves are accompanied by nerve-cells, which occur either singly or in groups.

Egorow describes the nerves as being distributed in three networks: one for the mucous membrane and muscular trabeculae; a second for the superficial muscular layer; and a third for the serous membrane.]

III. THE VOCAL SACS.

a. General description. The vocal sacs are a pair of sacs which open in the floor of the mouth (Fig. 179 S); they are found only in the males. When the animal croaks these sacs are dilated and act as resonators; when so dilated the sacs force up the skin under the angle of the mouth and tympanic membrane. In well-developed specimens they are about as large as an average sized cherry. The skin covering the sacs is extremely elastic, but is not directly attached to the sacs.

b. Minute structure. The sac consists of connective-tissue, with a large proportion of yellow elastic fibre. Internally it is lined with a flattened epithelium, and externally is covered with a layer of striated muscular fibre, derived from the mylo-hyoid muscle (Fig. 209 My, My¹).

IV. THE THYMUS GLAND.

a. General description. The thymus gland (Fig. 210 Th) is placed on each side behind the angle of the jaw; it is best exposed by removing the skin behind the tympanic membrane and the angle of the jaw, and then reflecting the M. depressor mandibulae (D.m.). The gland is then found as an elongated, oval body, not quite 3 mm. long, lying in the space between the M. depressor mandibulae and the M. sternocleidomastoideus (St); it extends slightly beyond the posterior border of the former muscle. The space also includes connective-tissue, fat, and numerous vessels.

In Rana temporaria this gland is spherical, much smaller, and placed further behind on the M. sternocleidomastoideus, between the M. latissimus dorsi and the M. deltoideus (Wiedersheim).

b. Minute structure (Fig. 211).

[The gland is surrounded by a connective capsule, which is indented on the inner surface to form a hilus through which blood-vessels course into the organ.

The capsule sends in numerous fine trabeculae, which form a connective-tissue skeleton such as is found in all lymphatic glands. The corpuscles of the trabeculae possess elongated nuclei from 0.019 to 0.028 mm. in length, and 0.010 to 0.015 mm. in breadth (Tolldt). The trabeculae support a network of blood-vessels.

The meshes of this sustentacular tissue are filled with cells; these are:

(α) Lymphoid cells, rounded or oval, possessing a round nucleus and nucleolus, and an extremely small amount of adhering protoplasm; the size of the nucleus is from 0.011 to 0.015 mm. (Tolldt).

(β) Corpuscles of Hassall (Fig. 211 II, III, and IV) are, as a rule, large bodies, but are subject to much variation in size. Their general appearance is seen in Fig. 211 III; they show a concentric striation and usually enclose one or more smaller cells. They therefore closely resemble similar corpuscles found in higher animals.

(γ) In many frogs the cellular structure of at least a part of the gland seems to have undergone a degenerative stage (III). In such cases the connective-tissue is increased in quantity, and marks off portions of the section into small lobules (III) which are filled with cells containing mucus or sometimes fat (III). Under what conditions this degeneration, if such it be, takes place has not yet been determined‍77.

(δ) Large branched pigment-cells are found in the course of the larger blood-vessels.

(ε) Watney describes also four varieties of ‘granular cells:’ 1. polygonal or rounded; 2. vacuolated; 3. spheroidal masses; 4. club-shaped masses attached to the blood-vessels. I have, however, not been able to distinguish them.]

V. THE THYROID GLAND.

a. General description. A thyroid gland (Fig. 209 Thy) is found on either side as a small, triangular, or oval, reddish-coloured body on the dorsal surface of the M. sternohyoideus, just before it passes between the MM. genio-hyoidei. It lies in the angle formed between the larger and smaller posterior cornua of the hyoid (HH, HH¹). It is easily found by the presence of a large number of vessels in its neighbourhood, and especially by the large jugular vein, to the ventral surface of which it is intimately attached.

Not uncommonly several smaller supplemental glands are found in the rich anastomosis surrounding the organ.

The dorsal surface of the gland is lobulated, the ventral surface flatter and convex. The glands of opposite sides are seldom symmetrical.

b. Minute structure (Fig. 212 I and II). [The gland possesses a connective-tissue capsule, which sends in trabeculae to support the vesicles of which the gland is composed.

The vesicles (I and II), which vary greatly in size, are closed cavities, usually of a rounded or oval form, but sometimes branched (Baber). Each vesicle is lined with a single layer of cubical or slightly columnar epithelium (II, a), which rests on a delicate basement-membrane of connective-tissue, placed between the epithelium and the surrounding lymphatics (Baber).

Zeiss describes a delicate reticulum between the epithelial cells.

The vesicles always contain mucus (b), and are surrounded by a fine anastomosis of blood-vessels (c).]

VI. THE LYMPHATIC GLANDS OF THE HYOID REGION (TONSILS?).

a. [General description. These are two oval, reddish-coloured, soft lymphatic glands, placed one on each side of the larynx. Frequently they are divided into two or three lobes by more or less deep fissures. They are constant in their occurrence, and are frequently supplemented by one or more smaller glands; each gland has the larynx on its inner side, the Vena jugularis externally, the M. omohyoideus in front, and above the mucous membrane of the pharynx (Tolldt).

b. Minute structure. The glands consist of dense lymphoid tissue (Fig. 213), but possess in addition one or more large bodies (a) which resemble lymphoid follicles in structure. Each is composed of a dense mass of small cells; and the whole follicle is, as in similar follicles of higher animals, sharply differentiated from the rest of the organ.

The glands possess an extremely rich vascular supply, and are frequently pierced by one or more large arterial trunks.

The mucous membrane covering the glands is thinner than that immediately around, but is not perforated.]

[As far as I am aware, Tolldt is the only observer who describes these glands; he makes no mention of the lymphoid follicles.]