pass from each hemisphere to its fellow through the corpus callosum, whilst others again bring the different convolutions of the same hemisphere into mutual connexion. The hemispheres are (so to speak) wrapped round the collection of Sensory Ganglia in which the spinal cord may be said to terminate at its upper end, in such a manner as to leave two cavities, one on either side, which are called the lateral ventricles.1 The Sensory Ganglia are so small relatively to the Cerebrum, that they would scarcely attract notice as independent centres, if they were not carefully compared with the ganglionic centres corresponding to them among the lower animals. The olfactory ganglia are mere bulbous enlargements upon the cords (1), which, though commonly termed the olfactory nerves, are really (as in the Shark, § 453) footstalks connecting these ganglia with the rest of the series; it being from these ganglia that the true olfactive nerves are given off (§ 506). The optic ganglia, g, only in part represent the optic lobes of Fishes; the function of the latter being shared by two large masses termed the thalami optici, which form the hinder part of the floor of the lateral ventricles, and which also seem to participate in the sense of touch, as the sensory columns of the spinal cord may be traced up to them. This close connexion of the sensorial centres of Sight and Touch is just what we might anticipate from the continual co-operation of these two senses (§§ 556, 557). In front of the optic thalami is another pair of large ganglionic masses, termed the corpora striata, which is in the like close relation with the motor columns of the spinal cord; and it is chiefly from them and from the thalami optici, that the fibres proceeding to the surface of the Cerebral hemispheres radiate. The Cerebellum, which has no direct communication with the Cerebrum, but possesses independent connexions of its own with the upper part of the spinal cord, has its grey or vesicular and its white or fibrous substance so peculiarly disposed, as to present in section the appearance delineated at d, which is termed the arbor vitæ, or tree of life.

459. Of the nerves given off within the skull (figs. 196,197),

1 There are other ventricles, which are merely spaces left on the middle plane by the imperfect coalescence of the two lateral columns of the nervous axis, like the openings formed by the divergence of the two halves of the nervous cord in Insects (fig. 188).

1

CEREBRO-SPINAL NERVES.

2

3

4

5

6

8

11

1 a b

371

5

9

e

10

d

14

13

15

16

17

9

the first pair are the olfactive, which proceed from the bulbs (1) of the olfactive peduncles, into the cavity of the nose. Next to these are the optic nerves (2), which may be partly traced to the optic ganglia, and partly to the thalami optici. The third (3), fourth (4), and sixth pairs (6), are nerves of motion only, and are distributed to the muscles of the eye. The fifth pair is for the most part a nerve of sensation only. Before leaving the skull, it divides into three great branches; of which the first (5) passes into the orbit (or cavity in which the eye is lodged), endows the parts contained in it with sensibility, and then comes out beneath the eyebrow, to be distributed to the forehead and temples; the second (5') passes just beneath the orbit, and makes its way out upon the face, supplying the cheeks, nose, upper lip, &c., which it endows with sensibility; whilst the third (5"), which (like the spinal nerves) possesses a motor root also, supplies the muscles of mastication with the power of moving, and the parts about the mouth with sensibility. The seventh pair (7), or facial, is the general motor nerve of the face; and this does not endow the parts which it supplies with the least sensibility. Beneath the origin of this nerve is seen the cut extremity of another trunk, that of the auditory nerve (8), or nerve of hearing. At 9 is seen the glosso-pharyngeal nerve, which supplies the back of the mouth and pharynx, and is concerned in the act of swallowing. Originating from the upper part of the spinal cord (or medulla bloongata) very near

k

j

23

Fig. 197.-BRAIN AND SPINAL
CORD OF MAN.

372

CEREBRO-SPINAL NERVES.

this, is the pneumogastric nerve, or par vagum (10), which supplies the lungs and air-passages, and also the heart and stomach. Below this, again, is the hypoglossal nerve (11), which gives motion to the tongue; at 12 is a nerve termed the spinal accessory, which is concerned in the acts of respiration; and at 13 and 14 are two of the regular spinal nerves. The termination of all these nerves is either in that prolongation of the Spinal Cord into the cavity of the skull, which is termed the Medulla Oblongata (fig. 197, f'), or in the Sensory Ganglia which are closely connected with the upper part of this prolongation. Although some of them seem to pass directly into the Cerebrum, it is very doubtful if such is really

the case.

460. A general connected view of the Brain and Spinal Cord is given in fig. 197; which represents the front of the latter, with the Brain a turned back, so as to expose its under side. At b is seen its anterior lobe; at c its middle lobe; and its posterior lobe d is almost entirely concealed by the Cerebellum e. At f' is shown the Medulla Oblongata, or upper end of the Spinal Cord ff. The brachial plexus is seen at g, formed by the nerves that originate in the cervical region of the cord; at h is the lumbar plexus formed by the nerves of the lumbar portion; and at k is the sacral plexus formed by the sacral nerves. The spinal cord terminates at its lower extremity in a bundle of nerves j, to which the name cauda equina is given, from its resemblance to a horse's tail. The various pairs of nerves from 1 to 14 are the same as in the preceding description; 15 and 16 are nerves from the upper part of the cervical region; 25, a pair from the dorsal region; and 33, a pair from the lumbar region. All these spinal nerves find their way out through apertures in the vertebral column, which are formed by a union of two notches, one in each of the adjoining vertebræ.

461. The system of nerves which has been now described is termed the Cerebro-Spinal; but it is not the only set of nerves and ganglia contained within the bodies of Vertebrated animals. In front of the vertebral column there is a chain of oblong ganglia, which communicate with two large ganglia that lie among the intestines, and with several small ganglia in the head and other parts. They communicate also with the posterior roots of the spinal nerves, on which are another set

SYMPATHETIC SYSTEM OF NERVES.

373

of ganglia (c, fig. 191), that seem to belong to the same system. The nerves proceeding from this system, which is called the Sympathetic, are distributed, not like those of the cerebrospinal, to the skin and muscles, but to the organs of digestion and secretion, and to the heart and blood-vessels. Hence the

former system of nerves, being that by which sensations are received and spontaneous motions executed, is called the nervous system of animal life; whilst the latter, being connected with the nutritive processes alone, is termed the nervous system of organic life.

462. What is the nature of the influence which the Sympathetic system exerts over the functions of the parts to which it is distributed, is not yet clearly made out. The sympathetic nerves distributed to the alimentary canal have been ascertained to have the power of exciting its peristaltic actions; and those which are distributed with the blood-vessels (on the coats of which they form a minute net-work) have a direct influence over their calibre, producing changes in the local circulation in obedience to passions and emotions of the mind, as well as to states of other bodily organs. Of this influence we have a familiar example in the acts of blushing and turning pale from agitation of the feelings, and a more decided but less frequent one in the fainting which sometimes occurs from a sudden shock. It is doubtful, however, whether the Sympathetic system really possesses motor filaments of its own; its motor actions being certainly in part dependent upon filaments derived from the cerebro-spinal system. The action of its motor fibres upon the muscular coats of the blood-vessels supplying the glands, serves to regulate the quantity of the fluids secreted by these organs, especially in cases in which the demand for the secretion is intermittent; but as there is evidence that the quality of many secretions may be affected by mental states (§ 353), it seems likely that the fibres peculiar to the Sympathetic system (§ 60) may be the channel of this influence.Although it is still impossible to define precisely the functions of the Sympathetic system, yet it may be stated generally, that in virtue of the two modes of action just explained, it seems to harmonise and blend together the various actions of Nutrition, Secretion, &c., in such a manner as to bring them into conformity with each other, and with the condition of the organs of Animal life.

374

FUNCTIONS OF SPINAL CORD:-REFLEX ACTION.

463. We shall now consider, in more detail, the functions of the different parts of the Cerebro-Spinal System in Man and the higher animals; referring occasionally to the Invertebrated classes for illustrations which they can best afford. We shall commence by examining the functions of the Spinal Cord and Medulla Oblongata, which are the parts concerned in reflex action.

Functions of the Spinal Cord.-Reflex Action.

464. The Spinal Cord of Vertebrated Animals may be considered as a collection of ganglia, analogous to those of which the ganglionic cord of Articulata is composed; these ganglia being united, however, in an unbroken line, instead of being distinct from one another and brought into communication by connecting cords. There is great difficulty in tracing-out the precise course of the nerve-fibres which form the white strands of the Spinal Cord; and it is doubtful how far any of them form a continuous connexion between the roots of the Spinal Nerves and the Brain. But there can be no doubt that such a connexion is established, either by the fibrous tracts or by the grey matter of the Spinal Cord; experiment having unequivocally shown that the latter participates with the former in this conducting power.

465. When the Cerebro-Spinal system is in full activity, its nerves convey impressions from every part of the body to the Brain, where they are communicated to the mind,—that is, the individual becomes conscious of them, or feels them as sensations. And by the fibres of the same system which pass in the contrary direction, the will acts upon the muscles so as to produce voluntary motion. Now the brain is not in constant action, even in a healthy person; it requires rest; and during profound sleep it is in a state of complete torpor. Yet we still see those movements continuing, which are essential to the maintenance of life; the breathing goes on uninterruptedly, liquid poured into the mouth is swallowed, and the position is changed when the body would be injured by remaining in it. The same is the case in apoplexy, in which the actions of the brain are suspended by pressure upon it. And the same will take place, also, in an animal from which the cerebrum has been removed; or in which its functions are completely suspended by a severe blow on the head. If