When this has been done, irritation of those sensory nerves which are connected with parts below the section excites the sensation of pain as strongly as ever. Hence it follows, that the afferent impulses, which excite pain when they reach the brain, pass through, and are conveyed by, the grey matter. And it has been found, by experiment, that, so long as even a small portion of the grey matter remains entire, these afferent impulses are efficiently transmitted. Singularly enough, however, irritation of the grey matter itself does not cause pain.

If one half of the cord, say the right, be cut through, transversely, down to its very middle, so as to interrupt all continuity of both white and grey matter between its upper and lower parts, irritation of the skin of the right side of the body, below the line of section, will give rise to as much pain as before, but all voluntary power will be lost in those muscles of that side, which are supplied by nerves coming off from the lower portion of the cord. Hence it follows, that the channels by which the afferent impulses are conveyed must cross over from the side of the cord which they enter to the opposite side; while the efferent impulses, sent down from the brain, must travel along that side of the cord by which they pass out.

If this be true, it is clear that a longitudinal section, taken through the exact middle of the cord, will greatly impair, if not destroy, the sensibility of both sides of the body below the section, but will leave the muscles perfectly under the control of the will. And it is found experimentally that such is the case.

15. Such are the functions of the spinal cord, taken as a whole. But particular regions of this

organ appear to be charged with the special function of acting as centres for those vaso-motor nerves,

The base of the brain.-A. frontal lobe, B. temporal lobe of the cerebral hemispheres; C.C. corpus callosum; Cb. cerebellum; M. medulla oblongata; P. the pituitary body; I. the olfactory nerve; II. the optic nerve; III. IV. VI. the nerves of the muscles of the eye; V. the trigeminal nerve; VII. the portio dura; VIII. the auditory nerve; IX. the glossopharyngeal; X. the pneumogastric; XI. the spinal accessory; XII. the hypoglossal, or motor nerve of the tongue. The number VI. is placed upon the Pons Varolii. The crura cerebri are the broad bundles of fibres which lie between the third and the fourth nerves on each side.

which supply the muscles of the vessels and of many of the viscera.

A side view of the brain and upper part of the spinal cord in place the parts which cover the cerebro-spinal centres being removed; CC. the convoluted surface of the right cerebral hemisphere; Cb. the cerebellum; M. Ob the medulla oblongata; B. the bodies of the cervical vertebræ; Sp. their spines; N. the spinal cord with the spinal nerves.

For example, the muscular walls of the bloodvessels supplying the ear and the skin of the head generally, are made to contract, as has been already mentioned, by nervous fibres derived, immediately, from the sympathetic. These fibres, however, do not arise from the sympathetic ganglia, but simply pass through them on their way from the spinal cord, to the upper dorsal region of which they can all be traced. At least, this is the only conclusion to be drawn from the facts, that irritation of this region of the cord produces the same effect as irritation of the vaso-motor nerves themselves, and that destruction of this part of the cord paralyses them.

The grey matter of the upper part of the cord is therefore a vaso-motor centre for the head and face.

16. The brain (Fig. 71) is a complex organ, consisting of several parts, the hindermost of which, termed medulla oblongata, passes insensibly into, and, in its lower part, has the same structure as, the spinal cord.

Above, however, it widens out, and the central canal, spreading with it, becomes a broad cavity, which (leaving certain anatomical minutiæ aside) may be said to be widely open above. This cavity is termed the fourth ventricle. Overhanging the fourth ventricle is a great laminated mass, the cerebellum (Cb. Figs. 71, 72, 73). On each side, this organ sends down several layers of transverse fibres, which sweep across the brain and meet in the middle line of its base, forming a kind of bridge (called Pons Varolii, Fig. 71) in front of the medulla oblongata. The longitudinal nerve-fibres of the medulla oblongata pass forwards, among, and between

these layers of transverse fibres; and become visible, in front of the pons, as two broad diverging bundles, called crura cerebri (Fig. 71). Above the crura cerebri lies a mass of nervous matter raised up into four hemispherical elevations, called corpora quadrigemina (C.Q. Fig. 73). Between these and the crura cerebri is a narrow passage, which leads from the fourth ventricle into what is termed the third ventricle of the brain. The third ventricle is a narrow cavity lodged between two great masses of nervous matter, called optic thalami, into which the crura cerebri pass. The roof of the third ventricle is merely membranous; and a peculiar body of unknown function, the pineal gland, is connected with it. The floor of the third ventricle is produced into a sort of funnel, which ends in another anomalous organ, the pituitary body (Pt. Fig. 73).

The third ventricle is closed, in front, by a thin layer of nervous matter; but, beyond this, on each side, there is an aperture in the boundary wall of the third ventricle which leads into a large cavity. The latter occupies the centre of the cerebral hemisphere, and is called the lateral ventricle. hemisphere is enlarged backwards, downwards, and forwards, into as many lobes; and the lateral ventricle presents corresponding prolongations, or

cornua.

Each

The floor of the lateral ventricle is formed by a mass of nervous matter, called the corpus striatum, into which the fibres that have traversed the optic thalamus enter (Fig. 73, C.S.).

The hemispheres are so large that they overlap all the other parts of the brain, and, in the upper view, hide them. Their applied faces are sepa