1911 Encyclopædia Britannica/Nervous System

by Frederick Gymer Parsons

NERVOUS SYSTEM. The nervous system forms an extremely complicated set of links between different parts of the body, and is divided into (A) the central nervous system, composed of (1) the brain, and (2) spinal cord; (B) the peripheral nervous system, consisting of (1) the cranial nerves, (2) the spinal nerves, (3) the various sense organs, such as the eye, ear, olfactory organ, taste organ and tactile organs, and (4) the motor end plates; (C) the sympathetic system. The anatomy and physiology of many of these parts are treated in separate articles (see Brain, Spinal Cord, Nerve, Eye, Ear, Olfactory Organ, Taste, Touch, Muscle and Nerve, Sympathetic Nervous System). The object here is to deal with anatomical points which are common to the whole system, or for which a place does not conveniently occur elsewhere.

Histology of the Nervous System.

Cunningham’s Text-
Book of Anatomy.

 Fig. 1.—Nerve-
fibre from a Frog.
(After v. Kölliker.)

Three kinds of tissue are found in the nervous system, nerve fibres, nerve cells, and a supporting tissue called neuroglia. Nerve fibres may be medullated or non-medullated, but, whichever they are, they consist of the long process or axon of a nerve cell; in a non-medullated nerve this process is either naked or enclosed in a delicate membrane called the primitive sheath or neurilemma but in a medullated nerve the process or axis cylinder is encased by a white. fatty substance called myelin, and so the term “myelinated” is often used instead of “medullated” for these nerves (see fig. 1). Outside this white sheath the neurilemma is present in most nerves, but is lost when they are massed to form the white matter of the central nervous system and in the optic nerve. At regular intervals the myelin is interrupted by some substance which stains deeply with silver nitrate, and these breaks are known as nodes of Ranvier. They do not, however, affect the axis cylinder. In a large nerve, such as the median, the nerve fibres are collected into small bundles called funiculi, enclosed in a connective tissue sheath, the perineurium, and separated from it by a lymph space. From this sheath delicate processes penetrate among the fibres, and these are known as the endoneurium. The funiculi are collected into bundles called fasciculi, and the whole nerve consists of a variable number of fasciculi surrounded by a dense fibrous sheath, the epineurium. The various bundles do not remain distinct, but break up and rearrange themselves, so that following them up with the scalpel is a difficult and tedious work. The nerve fibres, however, never join one another and are often several feet in length.

Nerve cells are unipolar, bipolar or multipolar. Unipolar cells are found in the ganglia on the posterior roots of the spinal nerves, and only give off an axon or axis cylinder process; this, however, soon divides in a T-shaped manner, and all these cells were originally bipolar, though the cell has grown away from its two axons (or, as they are often regarded, axon and dendrite), leaving a stalk joining it to them at right angles. Bipolar cells are found as an embryonic stage of unipolar, though in fish they persist in the spinal ganglia throughout life. They are also sometimes found in the sympathetic ganglia. Multipolar cells are found in the brain and cord, and are best studied in the anterior horns of the grey matter of the latter, where they are nearly visible to the naked eye (see fig. 2). Of their many processes only one is an axon, and it becomes the axial cylinder of a motor spinal nerve. The other fibres are called dendrites, and break up into delicate branches some of which surround, but, it is generally believed, are not actually continuous with, neighbouring cells or their processes. It is known that the axons are made up of delicate fibrils, and it is thought by some observers that there is actual continuity between some of these and those of an adjacent neuron, as the combination of a nerve cell, its axon and dendrites, is called. The cells of Purkinje in the cerebellum show a particularly rich arborization of dendrites (see Brain, fig. 7). Nerve cells have generally a large clear nucleus.

The Neuroglia is the delicate connective tissue which supports and binds together the nervous elements of the central nervous system. One part of it, which lines the central canal of the cord and ventricles of the brain, is formed of columnar cells, and is called ependyma, while the rest consists of small cells with numerous processes which sometimes branch and sometimes do not. These fibres interlace with one another to form a delicate felt-work which is unmixed with nervous elements on the surface of the grey matter of the brain (see Brain, figs. 7 and 15), though elsewhere it is interwoven with them.

Nerve Endings.—Sensory nerves end by breaking up into fibrillae or by various tactile organs. In the former case the minute fibrils, of which it has been shown that the axons or nerve fibres consist, separate and end among epithelial cells of the mucous membrane or skin. In the latter case the nerve fibres lose their coating of myelin and end in one of the seven following organs:—

1. End bulbs of Krause (fig. 3, A), oval bulbs composed of elongated cells among which the nerve fibrils end in knobs or coils; each is surrounded by a sheath of neurilemma, and the organs are found in the lips, tongue, conjunctiva, epineurium of nerves, synovial membranes of joints, and in the glans penis et clitoridis, where they have a mulberry-like appearance.

2. Pacinian corpuscles (fig. 3, B) are large enough to be seen by the naked eye, and are oval bodies made up of a series of concentric capsules of connective tissue rather resembling the structure of an onion; in the centre of this is a structureless core, at the distal extremity of which the nerve fibre ends in one or more knobs. These bodies are found in the palm and sole, in the mesentery, the genital organs and in joints.

Cunningham’s Text-Book of Anatomy.

Fig. 2.—Three Nerve-Cells from the Anterior Horn of Gray Matter
of the Human Spinal Cord.

3. Tactile corpuscles of Meissner and Wagner (fig. 3, C) are oval bodies found in certain of the skin papillae and mucous membrane, especially of very sensitive parts like the hand and foot, lips, tongue and nipple. They are oval and made of a connective tissue capsule from which septa enter the interior. The nerve fibre generally takes a spiral course through them, loses its myelin sheath, and ends by breaking up into its fibrils, which eventually become bulbous.

4. Tactile corpuscles of Grandry are found in the skin of those parts devoid of hair, and consist of a capsule containing two or more largish cells, between which the nerve fibre ends in the so-called tactile discs.

From Robert Howden, in Cunningham’s Text-Book of Anatomy.

Fig. 3.—Tactile Corpuscles. A, End bulb (Krause); B, Corpuscle
of Pacini; C, Corpuscle of Meissner. (B, C, after Ranvier.)

5. Ruffini’s endings are flattened oval bodies with a thick connective tissue capsule, in which the nerve fibre divides into many branches which have a varicose appearance, form a rich plexus, and end in knobs. These organs are found between the true skin and subcutaneous tissue of the fingers.

6. Organs of Golgi are found in tendons. Nerve fibres penetrate the tendon bundles and divide in a tree-like manner to end in little disks and varicosities.

7. Neuro-muscular spindles are small fusiform bundles of embryonic muscle fibres among which the nerve fibres end by encircling them and forming flattened disks. These are sensory endings, and must not be confused with the motor end plates. They are found in most of the striped muscles of the body.

Motor nerves end in striped muscle by motor end plates. These are formed by a nerve fibre approaching a muscle fibre and suddenly losing its myelin sheath while its neurilemma becomes continuous with the sarcolemma of the muscle fibre. The axis cylinder divides, and its ramifications are surrounded by a disk of granular matter containing many clear nuclei. In very long muscle fibres more than one of these end plates are sometimes found. Involuntary motor endings are usually found in sympathetic nerves going to unstriped muscle. The fibres form minute plexuses, at the points of union of which small triangular ganglion cells are found. After this the separate fibrils of the nerve divide, and each ends opposite the nucleus of an unstriped muscle cell.

The Sympathetic System

This system is made up of two gangliated cords running down one on each side of the vertebral column and ending below in the median coccygeal ganglion (g. impar). In the neck the cords lie in front of the anterior tubercles of the transverse processes of the cervical vertebrae, in the thorax, in front of the heads of the ribs, while in the abdomen they lie in front of the sides of the bodies of the vertebrae. In addition to these cords there are numerous ganglia and plexuses through which the sympathetic nerves pass on their way to or from the viscera and blood-vessels.

From A. M. Paterson, in Cunningham’s Text-Book of Anatomy.

Fig. 4.—Scheme of the Constitution and Connexions of the Gangliated Cord of the Sympathetic. The gangliated cord is indicated on the right, with the arrangement of the fibres arising from the ganglion cells. On the left the roots and trunks of spinal nerves are shown, with the arrangement of the white ramus communicans above and of the gray ramus below.

A typical ganglion of the sympathetic chain is connected with its corresponding spinal nerve by two branches called rami communicantes, one of which is grey and the other white (see fig. 4). The white consists of medullated fibres belonging to the central nervous system, and these are splanchnic afferent or centripetal, and efferent or centrifugal. The efferent fibres lie in the anterior roots of the spinal nerves, and, like all the fibres there, are either motor or secretory. They are the motor paths for the unstriped muscle of the vessels and viscera, and the secretory paths for the cells of the viscera. In the course of each fibre from the nerve cell in the spinal cord, of which it is an axon, to the vessel or viscus it supplies, there is always a break where it arborizes round a ganglion cell, and this may be in its own ganglion of the sympathetic chain, in a neighbouring ganglion above or below, or in one of the so-called collateral ganglia interposed between the sympathetic chain and the viscera. In addition to these there are a certain number of vaso-dilator and viscero-inhibitory fibres, which run without any cell connexions from the spinal or cranial nerve to the viscera. The splanchnic afferent or centripetal fibres are the sensory nerves from the viscera, and have no cell connexions until they reach the spinal ganglia on the posterior roots of the spinal nerves, which they do by traversing the gangliated cord of the sympathetic. The fibres of the white rami communicantes are remarkable for their small diameter, and the efferent fibres, at all events, are only found in two regions, one of which is called the thoracico-lumbar stream and extends from the first or second thoracic to the second or third lumbar nerve, while the pelvic stream is found from the second to the fourth sacral nerves.

	From A. M. Paterson, in Cunningham’s Text-Book of Anatomy.
	Fig. 5.—The Distribution of the Sympathetic Gangliated Cord in the Neck.
	Sy.1,	Superior cervical ganglion, and connexions and branches.
	I.C,	Internal carotid artery.
	G.Ph,	Glosso-pharyngeal.
	Hy,	Hypoglossal.
	C.1, 2, 3, 4, First four cervical nerves.
	Plex,	Pharyngeal plexus.
	G.Ph,	Glosso-pharyngeal nerve.
	E.C,	To external carotid artery.
	Sy.2,	Middle cervical ganglion, connexions and branches.
	C.5, 6,	Fifth and sixth cervical nerves.
	I.Thy,	Inferior thyroid artery.
	A.V,	Ansa Vieussenii.
	Sy.3,	Inferior cervical ganglion, connexions and branches.
	C.7, 8,	Seventh and eighth cervical nerves.
	Vert,	Vertebral plexus.
	Car,	Cardiac branches.

The grey rami communicantes are found in connexion with all the spinal nerves, though they are irregular in the paths by which they reach the sympathetic ganglia from the cells of which they spring; their fibres are mainly non-medullated, and pass into both roots of the spinal nerves and also into the anterior and posterior primary divisions of those nerves. In this way they reach the body wall and limbs, and are somatic vaso-motor secretory and pilo-motor fibres, supplying the vessels, glands and hair muscles of the skin and its glands. The sympathetic ganglia, from which these nerves come, contain multipolar nerve cells with one axon and several dendrites as well as a number of medullated fibres passing through, and much connective tissue. Some of the axons of these cells pass in the connectives to ganglia above and below, while others pass with the splanchnic efferent nerves to the viscera.

	From A. M. Paterson, in Cunningham’s Text-Book of Anatomy.
	Fig. 6.—The Arrangement of the Sympathetic System in the Thorax, Abdomen and Pelvis.
	T.1-12, L.1-5, S.1-5, Co,
		Anterior primary divisions of spinal nerves, connected to the gangliated cord of the sympathetic by rami communicantes, white (double lines) and gray (single lines).
	Oes,	Oesophagus and oesophageal plexus.
	Ao,	Aorta and aorta plexus.
	Va,	Vagus nerve joining oesophageal plexus.
	S.1,	Great splanchnic nerve.
	X,	Great splanchnic ganglion.
	S.2,	Small splanchnic nerve.
	S.3,	Least splanchnic nerve.
	Co,	Coronary artery and plexus.
	Spl,	Splenic artery and plexus.
	H,	Hepatic artery and plexus.
	SL,	Semilunar ganglion.
	Di,	Diaphragm.
	S.R,	Suprarenal capsule.
	Re,	Renal artery and plexus.
	S.M,	Superior mesenteric artery and plexus.
	Sp,	Spermatic artery and plexus.
	I.M,	Inferior mesenteric artery and plexus.
	Hy,	Hypogastric nerves and plexus.
	Rec,	Rectal plexus.
	Ut,	Uterine plexus.
	Ves,	Vesical plexus.
	V.V.V,	Visceral branches from sacral nerves.

The above sketch will give the general scheme of the sympathetic system, but its exact topographical details in man must be sought in the modern text-books such as those of Gray, Quain or Cunningham. Here only the larger and more important details can be given. In the gangliated chain there is a ganglion corresponding to nearly each spinal nerve, except in the neck, where only three are found; of these the superior cervical ganglion is more than an inch long, and is connected with the first four spinal nerves as well as with the ninth, tenth and twelfth cranial nerves (see fig. 5, Sy.1). Branches of distribution pass from it to the pharyngeal plexus, the heart and the two carotid arteries. Of these the branch accompanying the internal carotid artery passes to the carotid and cavernous plexuses, and through these communicates with the sphenomaxillary, otic and ciliary ganglia, while the branch to the external carotid communicates with the submaxillary ganglion. The middle cervical ganglion (fig. 5, Sy.2), when it is present, gives rami communicantes to the fifth and sixth cervical nerves, as well as branches of distribution to the thyroid body and heart.

The inferior cervical ganglion (fig. 5, Sy.3) lies behind the subclavian artery, and, besides the main connective cord, has a loop (ansa Vieussenii) joining it to the middle cervical ganglion in front of that vessel. It communicates with the seventh and eighth spinal nerves, and gives branches of distribution to the heart and to the subclavian artery and its branches, especially the vertebral. The thoracic part of the sympathetic cord has usually eleven ganglia, which receive both white and grey rami communicantes from the spinal nerves (fig. 6); of the former the upper ones run up in the chain and come off from the cervical ganglia as already described, while the lower ones form the three abdominal splanchnics which pass through the diaphragm (q.v.) and join the abdominal plexuses.

The great splanchnic (fig. 6, S.1) comes from the sixth to the ninth ganglia, and ends in the semi-lunar ganglion of the solar plexus ing. 6, SL). The small splanchnic (fig. 6, S.2) comes from the ninth and tenth, or tenth and eleventh ganglia, and ends in the aorticorenal ganglion of the solar plexus, while the smallest splanchnic (fig. 6, S.3) comes from the last thoracic ganglion, whether it be the tenth or eleventh, and ends in the renal plexus.

In the lumbar region the gangliated cord is very irregular; there may be four or more ganglia, and these are often fused. Grey rami communicantes are given to all the lumbar spinal nerves, and white ones are received from the first two. Most of the branches of distribution pass to the aortic plexus. The sacral gangliated cord runs down just internal to the anterior sacral foramina; it usually has four small ganglia, and the two cords end by joining the coccygeal ganglion or ganglion impar, though the two-fourth sacral ganglia are united by transverse interfunicular commissures. The white rami communicantes, already mentioned as the pelvic stream, from the second to the fourth sacral spinal nerves, do not enter the ganglia but pass directly to the pelvic plexuses (fig. 6, V).

Sympathetic Plexuses.—In the thorax are the superficial and deep cardiac plexuses and the coronary plexuses; the former receives the left superior cervical cardiac of the vagus, and lies in the concavity of the arch of the aorta. The deep cardiac plexus is larger, and lies in front of the bifurcation of the trachea; it receives all the other cardiac nerves, and communicates with the anterior pulmonary plexuses of the vagus (see Nerves: Cranial). The right and left coronary plexuses accompany the coronary arteries; the former communicates with both the cardiac plexuses, the latter only with the deep cardiac plexus.

In the abdomen the solar plexus is by far the most important. It lies behind the stomach and surrounds the coeliac axis; in it are situated the semilunar, aortico-renal and superior mesenteric ganglia, and from it are prolonged subsidiary plexuses along the main arteries, so that diaphragmatic, suprarenal, renal, spermatic, coeliac, superior mesenteric, aortic and inferior mesenteric plexuses, are recognized. The hypogastric plexus is the continuation downward of the aortic, and lies just below the bifurcation of the aorta (see fig. 6, Hy); it divides into two branches, which accompany- the internal iliac arteries and are joined by the pelvic stream of white rami communicantes from the sacral spinal nerves and some twigs from the ganglia of the sacral sympathetic to form the pelvic plexuses. These are prolonged to the viscera along the branches of the internal iliac artery, so that haemorrhoidal, vesical, prostatic, vaginal and uterine plexuses are found. By the side of the neck of the uterus in the last-named plexus several small ganglia are seen. (For the literature of the sympathetic system, see Quain’s Anatomy, London, 1895.)

Embryology of Nervous System

The development of the brain, spinal cord and organs of special sense (eye, ear, tongue), will be found in separate articles. Here that of the cranial and spinal nerves and the sympathetic system is dealt with. The thoracic spinal nerves are the most typical, and one of them is the best to begin with. In fig. 7, A the ganglion on the dorsal root (DR) is seen growing out from the neural crest, and the cells or neuroblasts of which it is composed become fusiform and grow in two directions as the ganglion recedes from the cord. Those which run toward the spinal cord are the axons, while those growing into the mesoderm are probably enlarged dendrites. The ventral roots (VR) rise as the axons of the large cells in the ventral horn of the grey matter, and meet the fibres of the dorsal root on the distal side of the ganglion (fig. 7, B). As the two roots join each divides into an anterior (ventral) and a posterior (dorsal) primary division (fig. 7, D), the latter growing into the dorsal segment of its muscle plate and the skin of the back. The anterior primary division grows till it reaches the cardinal vein and dorsal limit of the coelom, and there forks into a somatic branch to the body wall (fig. 7, C, So), and a splanchnic or visceral branch (fig. 7, C, Vi) which joins the sympathetic and forms the white ramus communicans. The somatic branch grows round the body wall and gives off lateral and anterior branches (fig. 7, E). In the limb regions the anterior primary divisions of the nerves divide into anterior and posterior secondary divisions, which probably correspond to the anterior and lateral branches of the thoracic nerves (fig. 7, E and F). These unite with neighbouring nerves to form plexuses, and divide again, but the anterior nerves keep to the ventral side of the limb and the posterior to the dorsal.

The cranial nerves are developed in the same way as the spinal, so far as concerns the facts that the motor fibres are the axons of cells situated in the basal lamina of the mesencephalon and rhombencephalon (see Brain), and the sensory are the axons and dendrites of cells situated in ganglia which have budded off from the brain. The evidence of comparative anatomy, however, shows that the cranial nerves cannot be directly homologized with the spinal, nor can the fact of there being twelve of them justify us in assuming that the head contains the rudiments of twelve fused or unsegmented somites. To this we will return later. The case of the optic nerve is different to that of any of the others. A. Robinson (Journ. Anat. and Phys., vol. 30, p. 319) has shown that most of its fibres are the axons of ganglion cells in the retina, and, as the retina is part of the optic vesicle and an outgrowth from the brain, the so-called optic nerve is only comparable to a tract of fibres within the brain.

From A. M. Paterson, in Cunningham’s Text-Book of Anatomy.
Fig. 7.—Development of the Spinal Nerves.
A, Formation of nerve roots.		C, Formation of nerves.
D.R,	Dorsal root.	So,	Somatic division.
V.R,	Ventral root.	Vi,	Visceral branch.
N.T,	Neural tube.	P,	Posterior primary division.
No,	Notochord.
Al.C,	Alimentary canal.	D, E, Formation of subordinate branches.
Ao,	Aorta.	Lat,	Lateral, and
V,	Cardinal vein.	Ant,	Anterior, branches.
M.P,	Muscle plate.
B, Formation of nerve trunk (N).		F, Formation of nerve trunks in relation to the limb; dorsal and ventral trunks corresponding to lateral and anterior trunks in D and E.
D.G,	Dorsal ganglion.
Sy,	Sympathetic cord.
W.D,	Wolffian duct.
Co,	Coelom.

The twelfth or hypoglossal nerve is regarded as a fusion of the motor roots of three spinal nerves, and embryology bears this out, for Froriep has described a small and transitory ganglion corresponding to the posterior root ganglion of this nerve. Another link in the chain of reasoning is that the first spinal or sub-occipital nerve often has its posterior root suppressed.

The sympathetic system is developed from the posterior root ganglia of the spinal nerves, by cells which in man migrate a few at a time. A. M. Paterson, however, believes that the sympathetic is developed, independently of the cerebro-spinal system, in the mesoderm (Phil. Trans. clxxxi. pt. B. p. 159). In embryos of 14·5 m.m. there are found masses of cells on each side of the abdominal aorta, permeated with blood vessels, and having the same structure as the carotid and coccygeal bodies. They are known as the organs of Zuckerkandl, and disappear soon after birth.

Comparative Anatomy

The comparative anatomy of the brain and spinal cord is dealt with in the separate articles devoted to them.

Spinal Nerves.—In Amphioxus the dorsal and ventral roots do not unite with one another but alternate, a dorsal root on one side being opposite a ventral on the other. The dorsal roots are both sensory and motor, the ventral only motor. In the Cyclostomata (Petromyzon) the arrangement is nearly the same, but in some regions there are two ventral roots to one dorsal. In the fishes and higher vertebrates. the dorsal and ventral roots unite, though in selachian (shark) embryos F. M. Balfour says that the dorsal and ventral roots alternate (The Development of Elasmobranch Fishes, London, 1878). When limbs are developed, beginning with fishes, limb plexuses are formed. Where the limbs are suppressed rudimentary plexuses may persist as in the snake, though usually they disappear.

The cranial nerves are only represented by two pairs in Amphioxus. In the Cyclostomata, fishes and Amphibia, ten pairs of nerves are found, which in their distribution do not always agree with those of man. In the Amniota or reptiles, birds and mammals, the eleventh and twelfth nerves have been added. The researches of W. H. Gaskell (“On the structure, distribution and functions of the nerves which innervate the visceral and vascular systems,” J. of Phys. vii. 1, 1886), O. S. Strong (“The cranial nerves of Amphibia,” J. Morph. x. 101), J. B. Johnston (J. Comp. Neurol. xii. 2 and 87), and others, show that the cranial nerves are formed of at least five components: (1) Ventral motor, (2) Lateral motor, (3) Somatic sensory, (4) Visceral sensory, (5) Lateral line nerves.

The ventral motor components are those which rise from cells situated close to the mid line, and probably correspond to the ventral roots of the spinal nerves. The nerves to the eye muscles (motor oculi, trochlearis and abducens) have this origin (see Nerve: Cranial), as also has the hypoglossal, which doubtless is a cephalized spinal nerve.

The lateral motor components rise from cells situated more laterally, and comprise the motor roots of the fifth (trigeminal), seventh (facial), and ninth, tenth and eleventh (glossopharyngeal, vagus and spinal accessory). These nerves supply muscles belonging to the branchial skeleton, instead of the muscles of the primitive cranium, of which the eye muscles are the remnants.

The somatic sensory components supply the skin, and end in cells which, among the cyclostomes and fishes, form a considerable elevation in the rhombencephalon, known as the lobus trigemini (fig. 8, Nuc. V.). These components, in the lower forms, are found in the fifth, seventh and tenth nerves, but in mammals practically only the fifth contains them. They correspond to the dorsal roots of the spinal nerves.

From Catalogue of the Museum of the Royal College of Surgeons of England, vol. 2, 2nd ed.

Fig. 8.—Brain (A) and Choroid Plexuses (B) of Lamprey.

The splanchnic sensory or viscero sensory components end in the brain in the medullary cells known as the fasciculus communis in fishes, and fasciculus solitarius in mammals (see Brain, fig. 4), as well as in the lobus trigemini and lobus vagi (fig. 8, Nuc. X). They are found in the fifth, seventh, ninth, tenth and eleventh nerves, and supply visceral surfaces. In mammals the lingual and palatine branches of the fifth, the chorda tympani and great superficial petrosal (?) of the seventh, and all the sensory fibres of the ninth and tenth except Arnold's nerve, represent these. In fishes. and Amphibians the palate' is supplied by the seventh nerve instead of the fifth, but the explanation given for this difference is that in these lower forms the Gasserian and geniculate ganglia are not distinct, and so fibres from the compound ganglion may pass into either nerve. These splanchnic sensory components of the cranial nerves evidently correspond to the branches which have already been mentioned as the splanchnic afferent fibres of the sympathetic.

The system of the lateral line or acustico-lateralis component is sometimes regarded merely as a subdivision of the somatic sensory. It is best developed in the fish, and may be divided into pre- and post-auditory, and auditory. The pre-auditory part comprises the pit and canal end organs supplied by the seventh, and also probable the olfactory organ supplied by the first nerve. The auditory apparatus, supplied by the eighth nerve, is, according to modern opinion, undoubtedly a part of this system, while the tenth nerve sends a large branch along the lateral line supplying the special end organs of the post-auditory part. All these components of the lateral line pass to the tuberculum acusticum in the fourth ventricle, as well as to the cerebellum, which J. B. Johnston (Zool. Bull. 1, 5, p. 221, Boston) regards as a derivative of the rostral (anterior) end of the acusticum. In mammals no doubt the olfactory and auditory apparatus and nerves have the same morphological significance as in fishes, but the seventh does not supply any cutaneous sense organs on the head or face, and the only vestige of the post-auditory supply of the tenth nerve to the lateral line is the small auricular branch of the vagus, often called Arnold’s nerve.

The following table, slightly modified from the one drawn up by J. McMurrich, gives a fair idea of the present state of our knowledge of the nerve components in the Mammalia.

Nerve		Ventral Motor.	Lateral Motor.	Somatic Sensory.	Splanchnic Sensory.	Lateral Line.
I.						＋ (?)
II.1
III.		＋
IV.		＋
V.			＋	＋	＋
VI.		＋
VII.			＋		＋
VIII.						＋
IX.	${\Bigg \}}$
X.			+	+	+	+
XI.
XII.		+
Spinal		+	(?)	+	+

¹ A tract of the brain.

For further details and literature of the nervous system see Quain’s Anatomy (latest edition); R. Wiedersheim’s Comp. Anat. of Vertebrates (Lond. 1907); Bronn’s Classen und Ordnungen des Thierreichs; C. S. Minot’s Human Embryology (1892); McMurrich’s Development of the Human Body (London, 1906). For the theory of nerve components see Onèra Merritt, Journ. Anat. and Phys., vol. 39, p. 199. A general discussion on the comparative anatomy and morphology of limb plexuses will be found in Miss C. W. Saberton’s paper on the “Nerve Plexuses of Troglodytes Niger” Studies in Anatomy, University of Manchester, vol. iii. (1906), p. 165. She refers to most of the literature on the subject, but the papers of H. Braus, Jena Zeitschr. v. 31 (1898), p. 239 on fish, of M. Davidoff, Morph. Jahrb. v. 5 (1879), p. 450 on the pelvic plexuses of fish, and of M. Fürbringer, Gegenb. Festschr. v. 3 (1897), on the spino-occipital nerves and brachial plexus of fish, are also very important.  (F. G. P.)