When sections are made through the corpus striatum, it is found to be traversed by very numerous bundles of fibres. It is necessary that these sections should be directed obliquely front below upwards in a direction parallel to the inferior layer of the crus cerebri. The bundles are thicker and more closely approxi mated to each other inferiorly; but as they ascend, they diverge, and radiate, some for-' wards, others outwards, and others backwards ; some pass nearly vertically upwards. A section made quite in the horizontal direction cuts all these fibres more or less transversely, so that the cut surface presents a grey colour inter spersed with white spots of variable size, ac cording as the bundles have been cut trans versely or obliquely ; but when the section is made in the oblique direction, as above di rected, then the surface presents a striated ap pearance like nunierous and regular white veins in a dark marble, the bundles of fibres being cut lengthways.
In tracing the bundles of fibres through the corpus striatum, we find that they divide and subdivide and occasionally anastomose. Each subdivision becomes clothed as it were with grey matter, which fills up the space between it and the adjacent ones. The grey matter en sheathes these bundles of fibres, as the areolar tissue does the fascicles of coarse muscles, and it may be dissected away from them, as we remove the areolar tissue from the muscular bundles.
It is an important problem to determine the exact source of these fibres and their precise destination. There can be no doubt that many of them are continuous with the inferior plane of the crus cerehri. Of those, the major part are usually supposed to pass through to the white substance of the hemisphere, and sorne no doubt proceed no farther than the corpus striatum. The other fibres which are found in this body may be viewed as taking their point of departure from its vesicular matter, and radiating, some outwards into the centrum ovale, others backwards to the optic thalamus, forming a bond of connection with that body. It must be borne in mind that, as the corpus striatum is a body of considerable thickness, these fibres which emerge from it must pro ceed in very different planes and with varying degrees of obliquity. Other fibres are found in the corpora striata, which however do not contribute to its striation. These are the fibres of the anterior commissure.
From a comparison of the small amount of fibrous matter in the inferior plane of the crus cerebri with the immense mass which forms the white substance of the hemispheres, (even if we exclude those fibres which form com missures,) it is impossible to suppose that the latter is derived from the former only ; nor, indeed, can it be admitted that even the greater part of the fibrous matter of the hemispheres is continuous with that of the crura, vvhether on their superior or inferior plane. A con
siderable portion of them doubtless, when traced from the hemispheres downwards, will be found not to pass below the corpora striata or optic thalamus.
We may regard the corpus striatum as a mass of grey matter with fibres implanted in it which connect it with the other parts of the encephalon. These parts are, 1st, the hemispheres ; 2d, the optic thalami ; 3d, the crura cerebri, mesocephale, and medulla ob longata. Of the?e last fibres it is probable, (but I am disposed to think far from certain,) that some of those which form the inferior layer of the crus pass through the corpora striata, and diverge among the other fibres of the centrum ovale.
Thus the corpora striata are connected to the optic thalami by fibres which pass from their concave or inner border to those bodies; to the convolutions of the brain by fibres continuous with some of those which form the white sub stance of the hemisphere, and we have seen that the convolutions of the insula have a very close relation to them; to the mesocephale and medulla oblongata by the fibres which form the inferior layer of the crus; and to each other by those which, emerging from them, contribute to form the corpus callosum, and also by the anterior commissure.
The vesicular matter of the corpora striata does not differ from that of the convolutions. It is traversed by a multitude of fibres. These, however, do not form any intricate interlace ment as in ganglia, but are collected into bun dles of very variable size; the largest being placed at the inferior part of the body, the smallest towards the hemispheres. The free or ventricular portion of the corpus striatum contains comparatively few fibres. NVhen a portion of the striated part of the body is examined under the microscope, the nerve fibres, of which the bundles are composed, ap pear to be reduced to their smallest size, and to be very compactly applied to each other, so that they transmit very little light, and therefore put on the appearance of dark cylin ders. It is only by very high powers that we can discover their fibrous structure. In many of the bundles the fibres appear to terminate at one extremity as if by forming an adhesion around a large vesicle, faint indications of the nucleus and nucleolus of which may be some times seen through the surrounding fibres. The appearance which the fibrous cylinders which exhibit this structure present calls to mind very strongly the representation of the nucleus of a comet with its tail. And this peculiarity of structure may be adduced as an argument that many if not the greater number of the fibres of the striated body form intimate connec tions with the elements of its vesicular matter.