I. THE CENTRAL NERVOUS SYSTEM.

(Re-written by the translator.)

A. The Spinal Cord (Fig. 97).

1. External form. The spinal cord is, in comparison with the brain, somewhat small; the two organs are directly continuous, and present no distinct line of demarcation: the point of origin of the first spinal nerve is, therefore, arbitrarily accepted as the anterior limit of the spinal cord; posteriorly it terminates in the filum terminale.

The spinal cord is flattened dorso-ventrally, and is constricted at a point (pars media, Reissner) somewhat anterior to its middle: in consequence of this constriction the cord has two enlargements; an anterior smaller, and a posterior larger (intumescentiae anterior v. posterior), from which arise the nerves of the brachial and lumbar plexuses respectively. At about the sixth or seventh vertebra, the hinder enlargement diminishes rather abruptly to form the so-called conus medullaris; this is continued into the filum terminate, which enters the cavity of the urostyle. The hinder portion of the lumbar nerves forms a cauda equina, the constituent nerves of which surround the filum terminale.

A dorsal longitudinal fissure (sulcus longitudinalis superior) is well marked in the middle line of the dorsal surface of the posterior enlargement; anteriorly and posteriorly it rapidly fades away, its position being merely indicated by a small amount of connective-tissue and a blood-vessel.

The ventral longitudinal fissure (sulcus longitudinalis inferior) is well marked throughout the length of the cord. Neither fissure can be traced in the hinder part of the conus medullaris or in the filum terminale.

Ten pairs of nerves arise from the spinal cord, each nerve arising by two roots, a ventral and a dorsal (anterior and posterior), from points near the ventral and dorsal longitudinal sinuses respectively: the two roots unite at their point of exit from the vertebral canal through the intervertebral foramen; each dorsal root possesses a ganglion.

2. Internal structure. As in other vertebrates, the spinal cord consists of white and grey matter, the latter being surrounded by the former; the relative amount of the one to the other varies in different parts of the cord.

A transverse section of the cord presents the same general characteristics as a similar section from a bird or mammal. The grey matter lies in the middle, surrounding the central canal, and is prolonged into each half of the section by ventral and dorsal horns or cornua. The grey matter is surrounded by the white, but is not so sharply marked off from this in amphibia as in birds and mammals.

A section through the anterior enlargement is almost quadrangular in outline, and somewhat broader below than above; the ventral longitudinal fissure is well marked and deep, while the dorsal is indistinct. The dorsal horns are narrow and short, the ventral longer and much broader. The space between the dorsal horns is narrow and deep, that between the ventral shallow; the outer boundary of the grey matter is almost straight, and the central canal is placed above the centre of the section.

In the constricted portion of the cord (pars media) the relative proportion of the white matter is increased at the expense of the grey; the horns are less distinct, the dorsal being directed outwards and the ventral somewhat flattened. The central canal is in the centre of the section.

The posterior enlargement shows the ventral horns projecting outwards, very much enlarged, and approaching the periphery; the space between them is semilunar. The dorsal horns are well developed, especially at their upper parts; the space between them is narrow and deep. The outer margin of the grey matter is again nearly a straight line. The central canal is below the centre of the section.

The arrangement of the parts in the conus medullaris approaches that in the pars media: the horns are much diminished in size, and posteriorly they entirely disappear, the ventral horns persisting longer than the dorsal. The central canal approaches the lower surface.

No cornua can be recognised in the filum terminale behind the origin of the last pair of spinal nerves. The grey matter has here a circular outline, with the exception of a very slight indentation below; the white matter is almost absent: the central canal is on the lower border of the section in the anterior part, while posteriorly it occupies almost the entire space below the pia mater.

The Central Canal lies in the median line, and always presents a distinct lumen; in the two enlargements this lumen forms a vertical ellipse‍51, while in the pars media it is circular. The canal is lined by a single layer of ciliated columnar epithelium, the cells of which are usually conical, with their bases directed towards the lumen; but other forms may be noticed. These cells average about 0·040 mm. in length and about 0·002 mm. in width at their broadest part. The peripheral extremities of the cells have processes, which, in the case of the cells above and below the cord, may often be traced as far as the pia mater; the processes of the lateral cells are less distinct and seem to be shorter. Anteriorly the central canal opens into the floor of the fourth ventricle. The termination of the central canal, posteriorly, has been described by Masius and Vanlair. According to these observers, the canal towards the hinder end of the filum terminale increases very gradually in width, until its epithelial cells are immediately underneath the pia mater, its walls then converge somewhat more sharply to close the cavity. From the point where the canal commences to increase in width, a change takes place in its epithelial wall, which, instead of having a single layer of cells, now possesses two to three layers; towards the posterior end it is again thinned out to a single layer.

The grey matter varies in different parts of the cord and in different parts of the same section. In a section through the anterior enlargement a portion of the grey matter, placed immediately above the central canal, is easily distinguished by its vertically-placed, elliptical outline, and by its transparency: it is known as the substantia reticularis. It consists of a mesh-work of branched cells, through which course distinctly contoured fine commisural fibres which are derived chiefly from the dorsal cornua, the vertical from the epithelial cells of the roof of the central canal. In a series of sections the substantia reticulosa may be seen to originate indistinctly in the most anterior part of the cord; in the anterior enlargement it already possesses the elliptical form mentioned, and in the pars media it commences to send off lateral processes to either side. In the posterior enlargement it reaches its greatest absolute development, as it here surrounds the central canal; while further backwards it encroaches until, together with the remaining grey matter, it constitutes almost the whole of the filum terminale.

Immediately underneath the central canal is a narrow band, which immediately connects the grey matter of the two sides; above it is bounded by the epithelium of the central canal, below by medullated fibres. This septum medium (Reissner) may be traced backwards from the anterior extremity of the cord to the posterior enlargement. It consists chiefly of transverse fibres, together with vertical fibres derived from the epithelial cells on the floor of the canal, and like the substantia reticulosa, is distinguished by its greater transparency from the rest of the grey matter.

The grey matter consists of connective-tissue and nervous elements, which are very intimately united; each of these elements including its special cells and fibres. The nerve-cells vary much in size and appearance. A prominent group of large cells in the ventral cornua, the lateral group (Stieda), is very conspicuous. The individual cells are seldom rounded, but are usually spindle-shaped or angular, and each possesses one to five processes, which may often be traced through considerable distances. These cells have an average length of 0·040 mm., and are about 0·016 mm. broad; they are somewhat larger in the anterior enlargement than in the posterior, and are also more numerous in a section from the anterior enlargement than in a section from the posterior. In the terminal filament they gradually disappear. Some of their processes are continued into the lateral columns (Köppen).

Smaller, spindle-shaped or triangular cells are scattered irregularly throughout the grey matter, without forming distinct groups, though for descriptive purposes those of the centre have been named the central group. In a stained, transverse section of the cord are seen numerous nuclei; these undoubtedly belong both to small nerve-cells and to connective-tissue cells, but except under the most favourable conditions the two are indistinguishable. The larger nerve-cells present the usual characters of nerve-cells: more or less granular contents, nucleus, well-marked nucleolus, more or less marked fibrillation; their processes are usually more homogeneous. The cells are frequently pigmented.

The processes of the cells belonging to the lateral group radiate in all possible directions, but certain well-marked processes directed towards the middle line can always be made out. From the lowest part of this group a few processes may be traced into the ventral roots. The small cells seem to be chiefly arranged vertically, although their processes radiate in all directions (Stieda).

The Fibres of the grey substance are nearly all non-medullated. They may be traced in all directions, but the best-marked groups are either vertical or transverse. The vertical fibres appear to arise from the central grey matter, and to ascend in larger or smaller bundles towards the periphery. The transverse fibres are arranged chiefly in the two commissures: the dorsal commissure (commissura superior) is the smaller; the fibres are parallel, and show no trace of decussation; externally they radiate in various directions to the dorsal horns. The ventral commissure is composed of two layers, the upper grey (commissura inferior grisea), the lower white (commissura inferior alba): both are interrupted by the septum medium. A well-marked decussation of the fibres is seen in the middle line; the exact mode of termination of these fibres has not been made out, but many appear to communicate with the large cells of the ventral horns.

The white matter consists chiefly of longitudinal, medullated fibres, in which various columns may be distinguished. The dorsal columns are separated from the lateral by a process of neuroglia, continued from the general investment of neuroglia lying under the pia mater. The line of separation between the lateral and ventral columns is not well marked; it is about the line which would be formed by prolonging the ventral horns to the surface.

The fibres vary in size, and fibres of all sizes may be found in any particular part of a transverse section; still the fibres of the ventral columns have an average greater diameter than those of the lateral columns, and the fibres of the dorsal columns are finer than those of the lateral. The largest fibres of the ventral column are placed near the ventral fissure and on its lower border; they attain their greatest development in the posterior enlargement; these fibres frequently contain two or three axis-cylinders each. Köppen suggests that they may represent the formatio reticularis of higher animals.

In the lateral columns the larger fibres are placed close to the grey matter, these columns also receive fibres from the cells of the lateral group.

In the dorsal columns the radiating root-fibres never reach the median plane, but leave an area of purely longitudinal fibres on each side of the dorsal fissure; these represent Goll’s columns, and have a club-shaped outline in transverse section.

The white matter is pierced in all directions by fine connective-tissue fibres and bundles of fibres which radiate from the grey matter; some branch and join with others to form a network, others pass almost uninterruptedly to the periphery. From the connective-tissue cells of the pia mater, processes pass into the white matter and assist in completing the connective-tissue matrix for the nervous elements.

Dorsal roots of the spinal nerves. Each root consists of a single bundle of nerve fibres, which suddenly bends at the periphery of the cord in order to descend vertically through the white matter towards the dorsal horn; the fibres divide into three sets, one to the dorsal commissure, a second to the upper horn itself, and the third helps to form the dorsal columns.

Ventral roots of the spinal nerves. Each ventral root consists of three or four delicate threads, which may be traced to the ventral cornua, which they reach after a vertical or very slightly oblique course through the white matter. Other fibres arise from the ventral columns, but these oblique fibres never extend to the median plane.

Pigment is found distributed irregularly through a section from any part of the spinal cord; it is increased in amount in those parts in which there is an increased amount of grey matter. The pigmentation is always found more marked in the lower parts of the ventral horns than in other parts.

B. The Brain (Figs. 98, 102, 103, and 105).

1. General description. From behind forwards, the dorsal surface of the brain presents the following parts for examination: the medulla oblongata, the cerebellum, the optic lobes, the thalamencephalon, the cerebral hemispheres, and the olfactory lobes.

The medulla oblongata is a direct continuation of the spinal cord; it is wider anteriorly than posteriorly, and is separated in front from the optic lobes by a vertical plate of nervous matter, the cerebellum. The optic lobes are two symmetrical ovoid bodies touching each other in the median plane, and together forming the widest part of the brain. In front of the optic lobes is the thalamencephalon, with a thick vascular membrane, the choroid plexus, lying on its upper surface, and connected with the pineal gland; the thalamencephalon extends forwards between the posterior ends of the cerebral hemispheres. The cerebral hemispheres are two symmetrical ovoid bodies, narrow in front, wider and slightly diverging behind: each hemisphere is prolonged forwards to form an olfactory lobe.

The ventral surface has in front the olfactory lobes, then the cerebral hemispheres, behind these the lamina terminalis, the tuber cinereum, the optic chiasma, the pituitary body, the crura cerebri, and lastly the medulla oblongata, in the order here given.

The various cranial nerves (Figs. 102 and 103) may be seen to arise as follows:—The olfactory nerve (I, I′, L.ol′) arises directly from the anterior end and outer side of the corresponding olfactory lobe, and from the cerebral hemisphere. The optic nerve (To and II) arises, as the optic tract, from the side of the brain below the optic lobe, whence it passes to the chiasma on the under surface of the brain. The oculo-motor (III) takes its origin from the ventral surface close to the median line and between the crura cerebri. The pathetic or trochlear nerve (IV) is attached to the dorsal surface between the optic lobes and the cerebellum. The trigeminal nerve (V) arises from the side and anterior part of the medulla oblongata. The abducens (VI) arises behind the pituitary body close to the median line from the ventral surface of the medulla halfway between the sulcus and the origin of the vagus. The facial and auditory (VII and VIII) nerves arise from the medulla oblongata behind the trigeminal nerve, the facial being in front of the auditory nerve. The glossopharyngeal (IX) nerve arises, in common with the pneumogastric nerve (X), behind the auditory nerve.

2. The several parts of the brain.

a. The medulla oblongata (Figs. 98, 102, and 103 Mo)
α. External form. The medulla is limited behind by the origin of the first pair of spinal nerves, at which point a very faint constriction is sometimes found: it extends forwards as far as the cerebellum. It gradually widens as it passes forwards until just before it reaches its anterior limit, where it presents a shallow but sharp constriction. The dorsal surface is characterised by the presence of a deep, triangular fossa, the fourth ventricle (Fig. 98 S.r), (ventriculus quartus, Stieda; sinus rhomboideus s. sinus triangularis, Reissner; fossa rhomboidalis); the sides of the triangle are, however, not quite straight, but are slightly bent outwards just before they converge towards the posteriorly-directed apex; the base of the triangle is formed by the cerebellum. By careful examination, the ventricle is seen to be continued for a short distance under the cerebellum, where it opens into the Sylvian aqueduct. In the floor of the fourth ventricle is a well-marked median longitudinal fissure (sulcus centralis), (Fig. 98 S). Into the posterior part of the ventricle opens the central canal of the spinal cord. As the fourth ventricle is formed by the white matter passing to either side, and the simultaneous flattening of the grey matter, the floor of the fourth ventricle is composed of grey matter.

The fourth ventricle is closed in by a highly vascular membrane, the choroid plexus of the fourth ventricle (plexus choroideus ventriculi quarti, Reissner; velum medullare posterius). The blood-vessels of the plexus will be described together with the other vessels of the brain (p. 162). They are supported by a connective-tissue matrix, and the whole covered with flattened epithelium, which in the fourth ventricle is ciliated and often pigmented.

The ventral surface of the medulla oblongata (Fig. 102 Mo) has a median ventral longitudinal fissure, a direct continuation of that of the cord; in the anterior part of the medulla oblongata there is also to either side of this a lateral fissure, continued on to the crura cerebri; these fissures correspond to the positions of the two rami posteriores of the internal carotid arteries; they are always well seen in microscopical sections. The medulla oblongata is so intimately connected with the pars commissuralis (pp. 149, 150) that the minute anatomy of the two is best described at the same time.

β. Internal structure. Examined by means of serial sections, the medulla oblongata is seen to have, in comparison with the cord, an increased amount of grey matter; this is especially the case in its anterior part. The floor and inner parts of the walls of the ventricle are formed of grey matter, in which the largest-sized cells have disappeared, to be replaced by medium-sized cells. Traced from behind, the ventral horns of the cord are seen to increase in size and to be more widely separated until they form two isolated masses, while the dorsal horns gradually diminish; at the same time they are forced outwards and upwards, until they lie under the floor of the ventricle, and so extend to the pars peduncularis.

Grey matter. The substantia reticularis is not present, but the septum medium extends forwards as far as the pars peduncularis. The central canal extends upwards at the expense of the tissue above it, and is here pear-shaped; at the same time the dorsal longitudinal fissure deepens until the two meet in the fourth ventricle; beyond this point one cannot speak of dorsal and ventral horns.

Small nerve cells are irregularly distributed throughout the whole of the grey matter and cannot be grouped; the larger cells, on the other hand, are arranged in distinct groups which have special relations with the nerves arising from the part. Occasionally these groups are not so isolated as usual; in this case processes of one group can be traced into another group (Reissner). Of these groups the chief are:

1. The nucleus centralis (upper inner group, Reissner), nucleus medullae oblongatae, Stieda (Fig. 99 i), is a group of cells found towards the hinder end of the medulla oblongata, on either side of and below the central canal; the group can be traced under the floor of the fourth ventricle to about its middle. The cells are rounded or spindle-shaped, the processes directed upwards, downwards, or outwards; their average size is 0·040–0·048 mm. long by 0·020 mm. broad.

2. The auditory nucleus (nucleus acusticus, Reissner, Stieda), (Fig. 100 n) is a large group of cells found in the wall of the fourth ventricle opposite the point of origin of the auditory nerve. The cells are rounded, pear-shaped, or of spindle form, and interspersed between the nerve fibres; these cells have an average length of 0·040 mm., and are about half as broad. The fibres of the auditory nerve radiate from their superficial origin in all directions through the grey matter towards these cells, and evidently communicate with them (Fig. 101 p). One small group (Fig. 101 r) passes to a lower level than the rest, and is regarded by Stieda as the true auditory centre. Köppen considers that the auditory nerve has a threefold origin: (1) from small cells on the median surface of the auditory area; (2) from the large cells between the above; (3) from a group of free nuclei on the dorsal surface of the auditory area.

3. The trigeminal nucleus (nucleus trigeminus), (Fig. 101 q) lies in part beneath the auditory nucleus but extends further forwards. It forms a rounded group of cells placed under the outer angle of the grey matter. The cells are somewhat crowded together, and are chiefly of an elongated spindle-form, with their processes directed obliquely downwards and outwards. The fibres of the trigeminal nerve separate into two groups; the upper group is best traced in a horizontal section, the fibres curving round to join the longitudinal fibres continued from the dorsal columns of the cord. The fibres of the lower, smaller group pass transversely inwards to the trigeminal nucleus. According to Reissner the latter fibres are motor, the former sensory. Probably other nerves are connected with the hinder part of this group.

4. The abducens nucleus (Fig. 100 o). From its superficial origin, the fibres of the abducens nerve may be traced vertically upwards to a small, rounded, grey mass; at this point the mass is somewhat isolated, but further forwards it may be traced as belonging to the central grey matter; it contains small spindle cells.

5. The pneumogastric nucleus. The pneumogastric, with its numerous irregular roots, arises from the side of the medulla oblongata. The hindermost fibres can be traced as a small bundle, passing almost transversely through the white matter to the outer margin of the grey matter. The larger portion of the fibres is placed in front of these; part of this seems to be directly continuous with the longitudinal fibres of the white matter; a second part, however, can be traced from the surface transversely through the white matter to the grey matter. These latter fibres, together with those of the group first described, do not arise from the grey matter in this part of the medulla oblongata, but curve round and run backwards longitudinally through the grey matter, thus forming a rounded bundle of fibres (Fig. 99 k). Between these fibres are interspersed small nerve-cells and nuclei which disappear as the fibres approach the white matter. The vagus undoubtedly receives fibres from the grey matter throughout a long course, and again receives a large bundle just before leaving the grey matter. The more exact origin of the various fibres has not been traced.

6. The nucleus magnus (Reissner and Stieda) is a very peculiar group of cells placed on either side, in the most anterior portion of the pars commissuralis, that is, immediately underneath the valvula cerebelli. The large cells are arranged in a transverse section in a single row so as to enclose a pear-shaped space on either side, which has its long axis directed from above, downwards and outwards, the narrower end being above. In longitudinal section the line of cells is seen to be open in front. The space enclosed by these cells is occupied by a granular ground-substance which contains only few nuclei. Bellonci is of opinion that these nuclei represent the corpora quadrigemina posteriora of higher animals.

White matter. In the hinder part of the medulla oblongata the arrangement of the white matter resembles that of the white matter of the spinal cord; further forwards the white matter of the dorsal surface commences to pass to either side, and ultimately it forms the outer part of the walls of the fourth ventricle. The fibres of the white matter of the ventral surface are unchanged in direction as they proceed forwards. The fibres of the anterior part of the medulla are thinner than those of the posterior portion (Stieda), according to Reissner they gradually thin as they pass forwards. The fibres are nearly all longitudinal, such transverse and oblique fibres as are present being chiefly in connection with the various nerve-roots and the commissures.

The commissura superior is naturally lost in consequence of the opening of the central canal into the fourth ventricle; the commissura inferior is increased in the anterior half and decreased in the posterior half of the medulla oblongata; in the latter the fibres become more and more oblique, and decussate very freely; ultimately they seem to be either continued as longitudinal fibres or to join the ganglia.

Near the pars commissuralis is a transverse arched band of fibres, passing from the under surface of one half of the cord over the ventral longitudinal fissure through the septum medium to the under surface of the opposite half; part of the fibres are continued upwards along the periphery to the cerebellum, part to the nucleus magnus. Vertical, straight, or slightly arched fibres are found in the walls of the fourth ventricle.

A section from the medulla oblongata has a larger amount of pigment than a section from the spinal cord, and the anterior portion of the medulla oblongata contains more than the posterior portion. The pigment is chiefly found in a curved line, placed in the lower and outer parts of the grey matter; the amount present varies in different specimens.

b. The Cerebellum and Valvula cerebelli.

α. External form. The cerebellum is a thin, semilunar plate, which projects between the optic lobes and the fourth ventricle, its base covering the most anterior part of the ventricle; the posterior surface possesses a very faint median fissure (Reissner). The valvula cerebelli (Velum medullare anterius, Reissner) is the thin lamella which connects the anterior surface of the cerebellum with the optic lobes.

β. Minute structure. By means of longitudinal, vertical sections, the posterior surface of the cerebellum is seen to be covered with epithelium; in the lower part of the surface this is columnar or conical, above it is flattened: immediately beneath, that is in front of this is a finely granular layer, with very closely packed and granular nuclei. In front of these is a stratum of nerve-fibres forming the second layer of the cerebellum.

Still more anteriorly is the third layer of the cerebellum, an irregular double layer of large cells (Purkinje’s cells, Denissenko); the cells have an average length and breadth of 0.040 mm. and 0.015 mm respectively; they are pear-shaped or of spindle-form, and possess usually two well-marked processes, one passing into the layer behind, the other forwards into the anterior layer to be immediately described, while other less distinct processes radiate irregularly in all directions. The fourth and most anterior layer of the cerebellum is a thick stratum of nerve fibres with numerous nuclei (0.006 to 0.008 mm. diameter). The fibres are for the most part arranged transversely, but some course in various directions. These fibres underlie the flattened epithelium which covers the anterior surface of the cerebellum.

The fibres of the second layer course, for the most part, in an almost vertical plane; they connect the cerebellum with the optic lobes (processus cerebelli ad corpora bigemina) and with other parts of the brain.

The fibres of the fourth layer receive numerous long processes from the large cells of Purkinje; they form a large commissural system, which can be followed ventrally on each side into the pars commissuralis. A part of the fibres ends here in the grey matter, a second portion enters the auditory area and forms a descending auditory root, a third part joins the lateral columns (in the medulla oblongata), and more anteriorly some join the ventral columns. The descending fibres from the cerebellum, together with the fibrae arcuatae found in the ventral columns, indicate the presence of a pons Varolii. The fibres of this ventral commissure decussate only on its dorsal surface (Köppen).

The Valvula cerebelli contains a few medullated fibres and the roots of the trochlear nerves; these pass from the medulla oblongata into the valvula cerebelli, cross in the median line, and then proceed forwards as the trochlear nerves.

The pigment in the pars commissuralis is arranged in a curved line similar to that found in the medulla oblongata, but the line is shortened at either extremity, and consequently does not extend into the cerebellum.

c. The optic lobes and Crura cerebri. (Corpora geminata and Pars peduncularis, Reissner; Lobus opticus, Stieda; Vierhügel, Tiedemann; Vierhügel (Zweihügel) and Pedunculi cerebri, Schiess; Mesencephalon, Huxley.)

α. External form (Figs. 98, 102, 103 L.op). The optic lobes are two prominent ovoid bodies placed immediately in front of the cerebellum, and connected with it by the valvula cerebelli; posteriorly they touch each other in the median plane, while anteriorly they diverge and thus constitute the widest part of the brain; in the angle thus formed is the thalamencephalon. The optic lobes are always more darkly pigmented than any other part of the central nervous system.

The Crura cerebri are two columns of white matter, placed beneath the optic lobes, and partly hidden by the pituitary gland. At their junction with the medulla oblongata, or rather with the pars commissuralis, is a very slight transverse fissure; at the same point the ventral longitudinal fissure is interrupted by an extremely small grey tubercle (Stieda).

β. Internal structure. From the anterior extremity of the fourth ventricle a canal, the Sylvian aqueduct (Aqueductus Sylvii, iter a tertio ad quartum ventriculum), may be traced forwards under the cerebellum, in the median line of this section of the brain. At about opposite the middle of the length of the optic lobes the canal is dilated and communicates with the cavities or ventricles (Ventriculi lobi optici, Stieda) enclosed by these; a general cavity is formed, which in transverse section has something of the form of the letter T. The roof of the cavity is thinner than the floor; this is especially the case in the median plane opposite the superior longitudinal fissure between the optic lobes; the floor is thinned in the middle line by the descending portion of the cavity. The cavity of each optic lobe extends both forwards and backwards beyond its point of communication with the dilated Sylvian aqueduct, hence in a transverse section taken in front of this point (Fig. 104 h′) the cavity of either side appears to be isolated; in a horizontal and longitudinal section (Fig. 105 Aq) the general arrangement of the parts may be well seen.