34 PHYSIOLOGY [NERVOUS Spinal cord. It is equally striking that the origin of the auditory nerves should be placed so far back as in the medulla oblongata and cerebellum, indicating the primitive nature of simple auditory impressions and their relation to co-ordination of movement. The sense of taste originates in nerves springing from the medulla, and in close con nexion with those regulating the movements of the tongue and swallowing. 3. When sensations of a simple character are elaborated into ideas and give rise to the physical changes in some way correlated to mental states, involving memory, emotions, volitions, and intel lectual acts, a part of the cerebro-spinal axis is differentiated for these functions in proportion to the extent to which such mental phenomena are manifested by the animal. Judging from the facts obtained by comparing animal intelligences, so far as they can be appreciated by us, we have the right to infer that in proportion to the degree of development in size and complexity of structure so is the mental condition of the animal. Taking it broadly, there can be no question that the intelligence of a bird is higher than that of a reptile, amphibian, or fish, and that the intelligence of the higher mammals, such as one of the Primates, is superior to that of the lower, as one of the Insect ivora (hedgehog), or of the Marsiqjialia (kangaroo) ; and along with the higher intelligence is the more complex brain. There are qualifications to this statement to be afterwards alluded to, but they arise from deficient knowledge and do not vitiate the main conclusion. In proportion, therefore, to the degree of development of the prosencephalon do we find the intelligence of the animal, and we may regard this portion as superadded to the cerebro-spinal axis as the organic mechanism for such mental operations. 4. There is also a correspondence between the degree of develop ment of the cerebellum and the faculty of co-ordination of move ment. Movements of the members of the body may be of a very simple character, or they may be very complex. They may be due to the action only of flexor and extensor muscles, causing the limb to move almost in the same plane, or they may be associated with the action of adductor and abductor muscles, by which there may be many kinds of circular or rotatory movements. There is a great difference between the movements of a fish s fin, of a bird s wing, of a horse s fore-leg, and of the arm of a monkey or a man. In the first three they are almost to-and-fro movements, unlike the deli cate movements of flexion, extension, pronation, stipulation, and prehension seen in the latter. Delicacy of movement of the anterior limb reaches its highest condition in man. It may be put generally that simplicity of movement is associated with an imperfectly- developed cerebellum, whilst in animals having the power of com plicated movements, involving especially the knowledge of how the limbs are acting at any moment, and of adjustment of movement in special circumstances, the cerebellum is highly developed. From this point of view, the degree of development of the cerebellum is as characteristic of man as the degree of development of the cere brum. That this is no accidental correspondence will be shown in treating of the functions of the cerebellum. Having reviewed the physiological anatomy of the cerebro-spinal system, an account will now be given of the more special physio logy of the centres composing it, namely, spinal cord, medulla oblongata, pons Varolii, basal ganglia (including corpora striata optic thalami, and corpora quadrigemiua), cerebellum, and hemi spheres of the cerebrum. Spinal Cord. The spinal cord is described at vol. i. p. 865 sq., but it is necessary here to allude to a few points of physiological importance. The cord consists externally of white and internally of grey matter. The _J"F. white matter, com posed of nerve-fibres, forms a series of strands or columns in each half of the cord. The grey matter in the central part of the cord is arranged in two crescentic masses, and shows under the microscope numerous multipolar cells con nected with nerve - fibres and imbedded in neuroglia, or the Fjo 21. Transverse section through spinal cord, connective AF, antero-median, and PF, postero-inedian fis sures ; PC, posterior, LC, lateral, and AC, anterior columns ; All, anterior, and PR, posterior nerve- roots ; C, central canal of cord, with its columnar epithelial lining. The crescentic arrangement of the grey matter is shown by the darker-shaded portion. cells having numerous branches called "Deiter s cells." (See fig. 21.) These nerve-cells are arranged in definite groups and occupy the same relative position in successive sections, forming the gan- glionic or vesicular columns of the grey matter, as follows. (1) Cells tissue of the nerve-cen tres. The neuroglia is composed of a kind of semi-fluid matrix, fibrils, and peculiar found along the whole of the anterior part of the anterior cornua, many of the processes of the nerve-cells being continuous with the nerve-fibres of the anterior roots of the spinal nerves. This column of nerve-cells has been called the " motor ganglionic column," or the "vesicular column of the anterior cornua." (2) A group or column of nerve -cells at the inner or mesial angle of the base of the posterior cornu, in the middle region of the cord from the third lumbar to the seventh cervical nerve. This is termed the " posterior vesicular column," or " Clarke s column," after the late Mr Loekhart Clarke, who did much to unravel the intricate anatomy of the nerve-centres. The nerve-cell processes are continuous chiefly with nerve-fibres coming from the lateral column. This vesicular column is best developed where the column of the anterior cornu is least so. (3) The third column of nerve-cells is in the outer most portion of the grey matter, midway between the anterior and posterior cornua. Development has shown that at an early period the anterior horns are distinctly differentiated from the posterior, and that the grey matter between them is the last to be formed. The nuclei in the latter may be regarded, therefore, as accessory nuclei. It has also been observed by Flechsig and others that the white substance of the cord also makes its appearance first in the neighbourhood of the anterior and posterior roots. The cord at a very early period consists almost entirely of grey matter, and the columns are superadded in the anterior first, the posterior last. The posterior can also be traced to the cortex of the cerebellum (Flechsig). The anterior and posterior roots of the spinal nerves are attached along the sides r? T> m f of the cord, op posite to the corresponding cornua of grey matter. Some of the fibres of the anterior roots end in nerve-cells in the anterior cornu. Others pass through the grey matter and cross to the other side of the cord through the ante rior commissure, a layer of white matter at the bottom of the anterior median fissure. A third set passes to the anterior part of the lateral col umn and to the F IO> 22. Diagram to illustrate the course taken by the posterior cornu. The course of libres of the nerve -roots on entering the spinal cord (Schafer ; Quain s Anatomy), a, a, two funiculi of an terior root of a nerve ; 1, 1, some of their fibres passing into lateral cells of anterior cornu ; 1 , 1 , others passing into mesial cells of same cornu ; 2, 2, fibres passing to lateral column of same side without joining nerve-cells ; these fibres is shown in fig. 22. A portion OI the 3 ; 3^ fji, res passing towards posterior cornu ; 4, 4, fibres fibres of the pos- passing across anterior commissure, to enter nerve-cells terior roots ends n anterior cornu of other side ; p, funiculus of posterior root; pi, fibres of its external or lateral division coming e b. t-y ma t- through and around gelatinous substance of Rolando ; some of these (5) are represented as becoming longi tudinal in the latter, others ((i, C) as passing towards Cl OSS to the crev anterior cornu, either directly or after joining cells in fi posterior cornu, and others (7) as curving inwards to- wards grey commissure ; ]nn, fibres of mesial or inner division, entering into posterior column and then be coming longitudinal ; p m , fibres from a posterior root which had joined the cord lower down and entered pos terior column, now passing into the grey matter at root of posterior cornu. Of these, 8 is represented as enter ing Clarke s column, 9 as curving round this and coursing to anterior commissure, and 10 as passing towards an terior cornu, the axis-cylinder processes of the cells of Clarke s column are shown arching round and taking The arrangement the direction of the lateral column ; amf, anterior median of the white or f s sure > P m /> posterior median fissure ; cc, central canal ; Sll, Bnbstaniia gelatinosa of Rolando, fibrous columns of the cord is seen in the table under medulla oblongata below. The spinal cord acts (1) as a transmitter of motor and sensory or centrifugal and centripetal impressions between the encephalon and the periphery, and (2) as a reflex centre. 1. Transmission of Motor and Sensory Imjrressions. Each spinal Motor nerve, as already mentioned, is connected with the spinal cord by and two roots, an anterior and a posterior. Section of a number ofsensorj anterior roots causes paralysis of motion of muscles on the same impres- side of the body, whilst irritation of the distal or peripheral end of sions. the divided roots causes twitch ings or tetanus of the muscles. Neither section nor irritation has any effect on sensation. Hence the anterior roots contain motor fibres, carrying impressions from ter on the same side, but many matter on opposite side. There is thus a decussation of fibres connected with both the anterior and the
posterior roots.