moved can smell, see, hear, and feel pain, to all appearance, as perfectly as, before (Flourens; Magendie). Yet, if any of its crura be touched, pain is indicated; and, if the restiform tracts of the medulla oblongata be irritated, the most acute suffering appears to be produced. It cannot, therefore, be regarded as a principal organ of sensation. (2.) Co-ordination of Movements. In reference to motion, the experiments of Longet and many others agree that no irritation of the cerebellum produces movement of any kind. Remarkable results, however, are produced by removing parts of its substance. Flourens (whose experiments have been confirmed by those of Bouillaud, Longet, and others) extirpated the cerebellum in birds by successive layers. Feebleness and want of harmony of muscular movements were the consequence of removing the superficial layers. When he reached the middle layers, the animals became restless without being convulsed; their movements were violent and irregular, but their sight and hearing were perfect. By the time that the last portion of the organ was cut away, the animals had entirely lost the powers of springing, flying, walking, standing, and preserving their equilibrium. When an animal in this state was laid upon its back, it could not recover its former posture, but it fluttered its wings, and did not lie in a state of stupor; it saw the blow that threatened it, and endeavoured to avoid it. Volition and sensation, therefore, were not lost, but merely the faculty of combining the actions of the muscles; and the endeavours of the animal to maintain its balance were like those of a drunken man. The experiments afforded the same results when repeated on all classes of animals; and from them and the others before referred to, Flourens inferred that the cerebellum belongs neither to the sensory nor the intellectual apparatus; and that it is not the source of voluntary movements, although it belongs to the motor apparatus; but is the organ for the co-ordination of the voluntary movements, or for the excitement of the combined action of muscles. Such evidence as can be obtained from cases of disease of this organ confirms the view taken by Flourens; and, on the whole, it gains support from comparative anatomy; animals whose natural movements require most frequent and exact combinations of muscular actions being those whose cerebella are most developed in proportion to the spinal cord. We must remember, too, that the cerebellum is connected with the posterior columns of the cord as well as with the direct cerebellar tract, both of which probably convey to the middle lobe muscular sensations. It is also connected with the auditory nerves. Movements of the eyes also occur on direct stimulation of the middle lobe. It seems, therefore, to be connected in some way with all of the chief sensory impulses which have to do with the maintenance of the equilibrium. Foville supposed that the cerebellum is the organ of muscular sense, i.e., the organ by which the mind acquires that knowledge of the actual state and position of the muscles which is essential to the exercise of the will upon them; and it must be admitted that all the facts just referred to are as well explained on this hypothesis as on that of the cerebellum being the organ for combining movements. A harmonious combination of muscular actions must depend as much on the capability of appreciating the condition of the muscles with regard to their tension, and to the force with which they are contracting, as on the power which any special nerve-centre may possess of exciting them to contraction. And it is because the power of such harmonious movement would be equally lost, whether the injury to the cerebellum involved injury to the seat of muscular sense, or to the centre for combining muscular actions, that experiments on the subject afford no proof in one direction more than the other. Forced Movements.-The influence of each half of the cerebellum is directed to muscles on the opposite side of the body; and it would appear that for the right ordering of movements, the actions of its two halves must be always mutually balanced and adjusted. For if one of its crura, or if the pons on either side of the middle line, be divided, so as to cut off from the medulla oblongata and spinal cord the influence of one of the hemispheres of the cerebellum, strangely disordered movements ensue (forced movements). The animals fall down on the side opposite to that on which the crus cerebelli has been divided, and then roll over continuously and repeatedly; the rotation being always round the long axis of their bodies, and generally from the side on which the injury has been inflicted. The rotations sometimes take place - with much rapidity; as often, according to Magendie, as sixty times in a minute, and may last for several days. Similar movements have been observed in men; as by Serres in a man in whom there was apoplectic effusion in the right crus cerebelli; and by Belhomme in a woman, in whom an exostosis pressed on the left crus. They may, perhaps, be explained by assuming that the division or injury of the crus cerebelli produces paralysis or imperfect and disorderly movements of the opposite side of the body; the animal falls, and then, struggling with the disordered side on the ground, and striving to rise with the other, pushes itself over; and so again and again, with the same act, rotates itself. Such movements cease when the other crus cerebelli is divided; but probably only because the paralysis of the body is thus made almost complete. Other varieties of forced movements have been observed, especially those named "circus movements," when the animal operated upon moves round and round in a circle; and again those in which the animal turns over and over in a series of somersaults. Nearly all these movements may result on section of one or other of the following parts; viz. crura cerebri, medulla, pons, cerebellum, corpora quadrigemina, corpora striata, optic thalami, and even, it is said, of the cerebral hemispheres. Sensory Centres in the Cerebral Cortex. Experimental lesions of various portions of the cerebral cortex and stimulation of such parts appears to show that the special senses are in some way represented at definite spots in the convolutions. Thus (a) the visual or optic centre is localised in the occipital lobe on either side on the outer convex part (fig. 358). This has been demonstrated in the dog's brain by Munk. In the humau brain there seems to be a very complex mechanism about this centre. The optic nerve-fibres having partially decussated in the chiasma pass in the optic tract to the optic thalami, and thence to the cortical substance of the occipital lobe. Hemianopia, restriction of the field of vision of opposite sides of the two eyes, may be produced, either by a lesion of one optic tract, in which are (chiefly) the crossed fibres from the nasal portion of the retina of the opposite eye and the uncrossed fibres of the external portion of the retina of the corresponding eye; or of the occipital centre. Part of the fibres of the optic tract pass to the corpora geniculata and to the corpora quadrigemina. Each of these so-called half-vision centres of opposite sides, situated in the occipital lobes, appears to be in connection with a higher centre in which the retina of both eyes are represented, but especially that of the opposite eye. If both occipital lobes be extensively diseased total blindness results. (b) The Olfactory centre, is said to be localized in the anterior extremity of the uncinate gyrus. The fibres, however, appear to be connected with a centre on the same side; others cross over to a centre on the opposite side. (c) The Auditory centre, is situated (according to Ferrier and Munk) in the monkey's brain in the first temporo-sphenoidal convolution. The auditory fibres pass up the pons in which they cross, and then in the superior portion of the tegmentum through the hinder portion of the internal capsule to this centre. Destruction of the entire region causes deafness of the opposite ear. (d) The centre for Taste has not yet been localised. According to Gowers, it is quite probable that the whole of the taste-fibres belong to the fifth nerve. Those which are distributed to the anterior parts of the tongue in the chorda tympani, coming from that nerve through the Vidian, which passes from the spheno-palatine ganglion to the facial, and those which are distributed to the back of the tongue through the glosso-pharyngeal, being derived from the otic ganglion of the fifth nerve through the small petrosal nerve and the tympanic plexus. CHAPTER XIX. PHYSIOLOGY OF THE CRANIAL NERVES. THE Cranial nerves are commonly enumerated as nine pairs; but the number is in reality twelve pairs, the seventh nerve consisting as it does, of two nerves, and the eighth of three. All arise Fig. 365.-Fourth ventricle, with the medulla oblongata and the corpora quadrigemina. The roman numbers indicate superficial origins of the cranial nerves, while the other numbers indicate their deep origins, or the position of their central nuclei. 8, 8', 8", 8", auditory nuclei nerves; t, funiculus teres; A, B, corpora quadrigemina; c, g, corpus geniculatum; p, c, pedunculus cerebri; m, c, p, middle cerebellar peduncle; s, c, p, superior cerebellar peduncle; i, c, p, inferior cerebellar peduncle; 1, c, locus cæruleus; e, t, eminentia teres; a, c, ala cinerea; a, n, accessory nucleus; o, obex; c, clava; f, e, funiculus cuneatus; f, g, funiculus gracilis. (superficial origin) from the base of the encephalon, in a double series which extends from the under surface of the anterior cerebral lobes to the lower end of the medulla oblongata. Traced into the substance of the brain and medulla, the roots of the nerves are found to take origin from various masses of grey matter, which are all connected one with another, and with the cerebral hemispheres. The roots of the olfactory and of the optic nerves have been already mentioned. The third and fourth nerves arise from grey matter beneath the corpora quadrigemina; and the roots of origin of the remainder of the cranial nerves can be traced to grey matter in the medulla oblongata in the floor of the fourth ventricle, and in the more central part of the medulla, around its central canal, as low down as the decussation of the pyramids. According to their several functions, the cranial nerves may be thus arranged: A. Nerves of special sense. B. Nerves of common sensation. D. Mixed nerves Olfactory, Optic, Auditory, part of The greater portion of the Fifth. Second, and Eighth will be The physiology of the First, considered with the organs of Special sense. The Third Nerve, or Motor Oculi. Functions. The Third nerve, or motor oculi, which arises in three distinct bands of fibres from the grey matter beneath the aqueduct of Sylvius near the middle line in conjunction with the fourth nerve. It supplies the levator palpebræ superioris musc'e, and all of the muscles of the eye-ball, but the superior oblique, to which the fourth nerve is appropriated, and the rectus externus which receives the sixth nerve. Through the medium of the ophthalmic or lenticular ganglion, of which it forms what is called the short root, it also supplies motor filaments to the iris and ciliary muscle. The fibres which subserve the three functions, accommodation, contraction of the pupil, and nerve-supply to the external ocular muscles, arise from three distinct groups of cells. When the third nerve is irritated within the skull, all those muscles to which it is distributed are convulsed. When it is paralysed or divided the following effects ensue :—(1) the upper eyelid can be no longer raised by the levator palpebræ, but |