The best-known long or extrinsic systems of fibres in the antero-lateral funiculus are those known as the fasciculus cerebrospinalis lateralis (O.T. crossed pyramidal tract), the fasciculus cerebrospinalis anterior (O.T. direct pyramidal tract), the fasciculus cerebellospinalis (O.T. direct cerebellar tract) (which goes from the spinal medulla to the cerebellum, and ought therefore to be called spinocerebellaris, as it will be subsequently named in this account), and the fasciculus anterolateralis superficialis (O.T. Gowers' tract).

There are, however, many other fasciculi at least as important as these, but there is as yet no close agreement as to their precise limits or connexions. One reason for this is that some of the elements of one tract may become intermingled with those of another; moreover, the position and relations of certain of them

FIG. 475.-DIAGRAM REPRESENTING THE CONNEXIONS OF SOME IMPORTANT SENSORY AND MOTOR TRACTS
IN THE BRAIN.

vary considerably at different levels of the spinal medulla. In Fig. 473 an
attempt has been made to present the present state of our knowledge of these
great strands of white fibres. This diagram is not intended to represent any definite
level of the spinal medulla, though certain features are shown which occur only
in the cervical region; and in respect of other features, the arrangement found.
in lower regions of the spinal medulla has been introduced to render the diagram.
more serviceable.

Much of the apparent complexity of this chart will disappear if the reader recalls some general statements (p. 512) made with regard to the outstanding features of the brain. It was then explained that when sensory nerves, coming from the skin and muscles, enter the spinal medulla, they not only establish relations with the motor nuclei and other spinal structures in the neighbourhood of their insertion, but also give rise, directly or indirectly (see Fig. 475) to many

fasciculi which pass upwards in the spinal medulla to reach the medulla oblongata, the pons and cerebellum, the mesencephalon (corpora quadrigemina), the thalamus, and the cerebral hemisphere. In the neighbourhood of each level where these ascending sensory tracts end, such as for example the region of the vestibular nucleus and cerebellum, the tectum mesencephali, the corpus striatum, and the cerebral hemisphere, great descending tracts originate and pass downwards in the spinal medulla (Fig. 475-the red lines). Thus we have cerebro-spinal, rubro-spinal, tecto-spinal, vestibulo-spinal, and bulbo-spinal fasciculi passing down the spinal medulla; and each system eventually ends around the series of motor nuclei (Fig. 475), many of them in the spinal medulla.

In the anterolateral funiculus the various fasciculi will be found to be grouped roughly into three bands:-Next to the gray columns is the fasciculus proprius; then comes a band of descending (motor) fasciculi; and then, upon the surface, a series of ascending (sensory) fasciculi. This arrangement, however, is not maintained with any degree of exactitude in the anterior funiculus, where the sharp demarcation between ascending and descending fasciculi is in great part destroyed by the intermingling of fibres passing in opposite directions.

The fibres of the posterior nerve-root have already been studied so far as their relation to the posterior funiculus is concerned. No clear conception of the nature and significance of the ascending fasciculi in the anterolateral funiculus can be obtained unless they also are studied in relationship with the fibres of the posterior root. It has already been explained that of the fibres which enter the spinal medulla in the posterior root the great majority enter the posterior funiculus, where they bifurcate (Fig. 473, a); one branch of each fibre passes upwards either in the funiculus gracilis or in the funiculus cuneatus, or it may pass from the latter into the former; the other descends in the fasciculus interfascicularis (O.T. comma tract). Other fibres perhaps enter the posterolateral fasciculus (O.T. Lissauer's bundle). But all the other fibres of the posterior root, together with the majority of the fibres of the fasciculus cuneatus, sooner or later enter the gray matter (Fig. 473, b to h) of the spinal medulla.

Some of them (b) pass directly to end in the nucleus dorsalis of their own side, and from its cells fresh fibres arise, which pass laterally through the posterior column and lateral funiculus to reach the surface, where they bend upwards as constituent fibres of the spino-cerebellar fasciculus. These pass upwards throughout the whole length of the spinal medulla (above their place of origin), into the medulla oblongata, thence into the cerebellum through the restiform body.

Other fibres on the same side (c), and perhaps also on the other side (d), end amidst cells of the gray matter, the axis-cylinder processes of which pass into the antero-lateral superficial fasciculus (O.T. Gowers' tract). In this tract they ascend throughout the spinal medulla, medulla oblongata, and pons, to enter the cerebellum alongside the brachium conjunctivum (superior peduncle). This element in the antero-lateral fasciculus is sometimes designated the fasciculus spinocerebellaris anterior, to distinguish it both from the non-cerebellar fibres of the parent fasciculus and from the fasciculus spinocerebellaris [posterior] (O.T. the direct cerebellar tract). These two spino-cerebellar tracts convey to the cerebellum information from the muscles and overlying skin which assists it to co-ordinate the muscles for carrying on precisely adjusted movements.

Other fibres of the posterior nerve-root (e, f, g, and h) terminate in relationship with cells in the gray columns of their own side of the spinal medulla, the axons of which cross the median plane in the anterior commissure to pass respectively (e) into the anterolateral superficial fasciculus [not to be confused with the cerebellar constituents of this bundle]; (f) into the real fasciculus spinothalamicus [posterior], of which the last-mentioned fibres are merely outlying members; (g) into the fasciculus spinotectalis, to ascend to the mesencephalon; and (h) into the marginal area of the anterior funiculus to form a group which may be called the fasciculus spinothalamicus anterior.

The careful investigations of the late Dr. Page May led him to attach a definite physiological significance to this grouping of the ascending paths. The fasciculus spinothalamicus [posterior] is supposed to convey upwards to the thalamus (for

transmission to the cerebral cortex, which is concerned with the conscious appreciation of sensations) all impulses of pain, heat, and cold coming from the skin upon the opposite side of the body. The fasciculus spinothalamicus anterior conveys impulses of touch and pressure from the opposite side.

The spino-cerebellar fasciculi [anterior and posterior] convey to the cerebellum respectively homolateral and bilateral unconscious afferent impulses underlying muscular co-ordination and reflex tone.

Among the descending tracts that establish connexions between various parts of the brain (see Fig. 475) and the motor nerve-cells in the anterior column may be mentioned the cerebrospinal, the rubro-spinal (from the red nucleus), the tectospinal (from the corpora quadrigemina), the vestibulo-spinal (from the terminal nucleus of the vestibular nerve), and the bulbo-spinal tracts. The last-mentioned forms a peculiar triangular area upon the surface immediately to the lateral side of the anterior nerve-roots (Fig. 473), but there is great uncertainty as to its mode of origin: it is often called the fasciculus olivospinalis, from the fact that its discoverer, Helweg, believed it to originate from the olivary nucleus in the bulb or medulla oblongata. It may be regarded as an outlying part of the vestibular (or cerebellar) tract to the motor nuclei of the spinal medulla.

The fasciculus cerebrospinalis lateralis (O.T. crossed pyramidal tract) is a large well-defined descending tract which lies immediately in front of the posterior column of gray matter, and subjacent to the posterior spino-cerebellar fasciculus, which shuts it out from the surface. Below the point where the posterior spino-cerebellar fasciculus begins the cerebrospinal fasciculus becomes superficial, and in this position it can be traced as low as the fourth sacral nerve, at which level it ceases to exist as a distinct strand. The cerebro-spinal fasciculus is composed of an admixture of both large and small fibres. These arise in the brain from the large pyramidal cells of the motor or precentral area of the cerebral cortex, and pass downwards through various subdivisions of the brain to gain the spinal medulla. As they enter the spinal medulla they cross the median plane from one side to the other, and it thus happens that the cerebro-spinal tract in the right lateral funiculus of the spinal medulla has its origin in the cortex of the left cerebral hemisphere, and vice versa. As the tract descends in the spinal medulla it gradually diminishes in size; and this is due to the fact that, as it traverses each spinal segment, numerous fibres leave it to enter the anterior column of gray matter, and end in connexion with the anterior motor cells from which the fibres of the anterior nerve-roots arise. The entire strand is ultimately exhausted in this way. Numerous collateral fibrils spring from the cerebro-spinal fibres, and, entering the gray matter, end in a similar manner. In this way a single cerebro-spinal fibre may be connected with several spinal segments before it finally ends. The lateral cerebro-spinal fasciculus must be regarded as a great motor strand which brings the spinal motor apparatus under the control of the will.

Schäfer believes that many of the fibres of the cerebro-spinal fasciculus end in connexion with the cells of the nucleus dorsalis.

In many marsupials, rodents, and ungulates the lateral cerebro-spinal fasciculus lies in the posterior funiculus of the spinal medulla.

The fasciculus lateralis proprius represents the remainder of the lateral funiculus. Its fibres are largely derived from the cells situated in all parts of the gray matter, and also from the nerve-cells of the opposite side of the spinal medulla. After a course of very varying length in the fasciculus lateralis, these fibres turn medially and re-enter the gray matter. Such fibres may thus be regarded as inter-segmental association fibres binding two or more segments of the spinal medulla together. It may be mentioned that the association fibres which link together segments of the spinal medulla which are near to each other lie close to the gray matter, whilst those which connect the more distant segments are situated further out in the lateral funiculus.

Funiculus Anterior.-One well-defined tract is situated in the funiculus anterior. This is termed the fasciculus cerebrospinalis anterior. The remainder of the funiculus receives the name of the fasciculus anterior proprius.

The fasciculus cerebrospinalis anterior (O.T. direct pyramidal tract) is usually a nerve-strand of small size which lies near the anterior median fissure. As a rule it cannot be traced lower than the middle of the thoracic region of the spinal medulla. It is a descending tract and must be associated with the lateral cerebro-spinal fasciculus of the opposite side, seeing that both of these strands arise from the motor area of the cortex of the same cerebral hemisphere. From this it must be clear that the anterior cerebro-spinal fasciculus does not cross the median plane as it enters the spinal medulla, but descends on the side of the spinal medulla corresponding to the cerebral hemisphere in which it arises. Nevertheless, its fibres do not end in the same side of the spinal medulla, but at every step along the path of the strand they make use of the anterior commissure, and cross to the opposite side of the spinal medulla, to terminate in relation to the opposite ventral motor cells in the same manner as the lateral cerebro-spinal fibres.

From this crossing of the cerebro-spinal fasciculi, it follows that the destruction of the fibres which compose them as they descend in one side of the brain must result in paralysis of the muscles supplied by the efferent nerves of the opposite side of the spinal medulla.

In cases of old brain lesion it is sometimes possible to detect some degenerated fibres in the lateral cerebro-spinal fasciculus of the sound side of the spinal medulla, and from this it is supposed that this tract contains a few uncrossed fibres. If this is the case, each side of the spinal medulla stands in connexion with the motor area of both cerebral hemispheres.

It is well to note that the fibres of both lateral cerebro-spinal fasciculi are not medullated until the time of birth. They are the latest of all the fasciculi of the spinal medulla to myelinate.

Commissura Anterior Alba.-The anterior white commissure is composed of medullated nerve-fibres passing from one side of the spinal medulla to the other and entering the anterior column of gray matter, and also the anterior funiculus of white matter. It is to be regarded more as a decussation than as a commissure, and its width, which varies somewhat in different regions, fluctuates in correspondence with the diameter of the spinal medulla.

Amongst the fibres which cross in the anterior commissure may be mentioned: (1) The fibres of the fasciculus cerebrospinalis anterior; (2) collaterals from both the anterior and lateral funiculi; (3) axons of many of the cells of the gray matter; (4) the dendritic processes of some of the medial anterior cells.

Commissura Grisea. Although this is composed of gray matter with a large admixture of neuroglia, numerous nerve-fibres pass transversely through it, so as to establish relations between the cells in the gray matter on the two sides of the spinal medulla.

THE ENCEPHALON OR BRAIN.

The brain is the enlarged and greatly modified upper part of the cerebro-spinal nervous axis. It is surrounded by the same membranes that envelop the medulla spinalis (viz., the dura mater, the arachnoid, and the pia mater), and it almost completely fills up the cavity of the cranium. So closely, indeed, is the skull capsule moulded upon the brain that the impress of the latter is almost everywhere evident upon the inner surface of the cranial wall. The relations, therefore, of cranium to brain are totally different from those presented by the vertebral canal to the spinal medulla. As we have noted, the medulla spinalis occupies only a part of its bony case; and there is not only a wide and roomy space between the arachnoid and the pia mater, but also an interval of some width between the dura mater and the walls of the vertebral canal.

General Appearance of the Brain.-When viewed from above the brain. presents an ovoid figure, the broad end of which is directed backwards. Its greatest transverse diameter is usually found in the neighbourhood of that part which lies between the two parietal tuberosities of the cranium. The only parts which are visible when the brain is inspected from this point of view are the two convoluted cerebral hemispheres. These present an extensive convex surface, which

is closely applied to the internal aspect of the cranial vault, and are separated from each other by a deep median cleft, termed the fissura longitudinalis cerebri, which extends from the front to the back of the brain.

The inferior aspect of the brain is usually termed the basis cerebri. It presents an uneven and irregular surface, which is more or less accurately adapted to the inequalities on the floor of the cranial cavity. Upon this aspect of the brain some of its main subdivisions may be recognised. Thus, posteriorly, is seen the short cylindrical portion, called the medulla oblongata, through which, at the foramen magnum, the brain becomes continuous with the medulla spinalis. The medulla oblongata lies on the ventral aspect of the cerebellum, and occupies the vallecula or hollow which intervenes between the two cerebellar hemispheres. The cerebellum

FIG. 476.-THE BASE OF THE BRAIN WITH THE CEREBRAL NERVES ATTACHED.

is a mass of considerable size which is placed below the posterior portions of the two cerebral hemispheres. It is easily recognised on account of the closely set, curved, and parallel fissures which traverse its surface and give it a foliated appearance. Above the medulla oblongata, and in close connexion with it, is a prominent white elevation called the pons. Immediately in front of the pons there is a deep hollow or recess. This is bounded behind by the pons, on each side by the projecting temporal lobe of the cerebral hemisphere, and in front by the orbital portions of the frontal lobes of the cerebral hemispheres. Passing out from each side of the anterior part of this recess is the deep lateral fissure of the brain which intervenes between the pointed and projecting extremity of the temporal lobe and the frontal lobe of the cerebrum, whilst, in the median plane in front, the longitudinal fissure, which separates the frontal portions of the cerebral hemispheres, opens into it.

Within the limits of this deep hollow, on the base of the brain, two large rope