THE INTERNAL CAPSULE.

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greater part of the so-called "corona radiata," which were described in a previous page; although, properly speaking, the internal capsule ceases at the optic tracts below and the upper level of the lenticular nucleus above. If we trace the anterior fibers of this bundle from below upward, we shall see that it divides the corpus striatum of each hemisphere into its two portions, the caudate and lenticular nuclei. The posterior fibers of the internal capsule separate the lenticular nucleus from the optic thalamus of the corresponding side (Fig. 38). The diagram, to which I now direct your attention, will make the relations of this bundle apparent, while it will also show the peculiar angle or bend which the internal capsule exhibits in all horizontal sections of the brain which intersect the basal ganglia. The fibers that form the "caudo-lenticular" portion of the capsule are imperfectly understood. We have no positive evidence of their motor function. The pyramidal tract lies posterior to the “genu of the capsule, as do also the motor fibers of the face. Back of these tracts we encounter the general sensory tracts. The optic fibers appear to lie still farther back, passing to the occipital cortex (see Fig. 39).

Again, if a cross vertical section of the cerebral hemispheres be so made as to include the substance of the thalamus and the lenticular nucleus, it will be perceived that the peripheral outline of these two masses of gray matter may be compared to a square; and that a diagonal band running from the outer and upper corner to the lower and inner corner of this square corresponds to the situation of the "internal capsule," which is included between these ganglia. Above the level of the basal ganglia, the fibers of the internal capsule radiate to join certain convolutions or "gyri" which will be enumerated later. Thus it is that the fibers which compose the internal capsule appear in most of the crosssections of the middle zone of the cerebrum to bear a fancied resemblance to the handle and sticks of a Japanese fan; the handle being the constricted portion between the corpus striatum and the optic thalamus, or the capsule itself, and

the diverging fibers being located within the medullary center of the cerebral hemisphere.

The extension of sensory fibers from the tegmentum cruris upward within the internal capsule of the cerebrum is now as clearly proved as is the continuity of the motor tract anteriorly. The course of the former has been studied by dissec

capsule.

a, the portion which lies anteriorly to the knee of the capsule. The functions of the fibers which compose this portion are not, as yet, definitely determined. f, the fibers for the face; m, the fibers of the so-called "motor tract"; s, those of the sensory tract"; s, t, those of the "speech tract"; o, those of the "optic tract." The fibers of each tract are probably associated with both sides of the body, but chiefly with the opposite side.

tion, embryological investigation, physiological experiment, and, finally, by the examinations of pathological processes. It has been shown by Türck' that, when certain convolutions of the brain (chiefly those which are motor in function) have suffered partial or complete destruction, that a descending' degeneration follows the course of the nerves which are con

1 This author first made known his great discovery to the Academy of Sciences of Vienna, in 1851.

Degeneration of nerves follows, as a rule, the direction of the currents which are normally conveyed by them. By this means, the question of the afferent or efferent character of certain nerve-tracts has been positively decided.

METHODS OF TÜRCK AND WALLER.

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nected with the cells of the injured part. This degenerative process extends along the nerves, from the cells of the cortex, to their peripheral terminations, in the cells of the spinal gray matter; thus enabling a careful observer to trace the paths of the fibers with even greater accuracy and positiveness than the most skillful dissector could possibly hope to attain. By means of this fact,' amplified somewhat by Waller and Gudden, physiologists have been enabled to solve many problems regarding the origin and course of special nerves, as well as certain nerve-tracts within the spinal cord and brain, which could not otherwise have been determined.

Although the remarkable observations of Türck were given to the profession some years before Waller was awarded the honor of meriting recognition as the recipient of the Moynton Prize for Experimental Physiology, his paper remained comparatively unknown for some years, when its great value at last became recognized.

The difference between the discoveries of Waller and Türck lie in the fact that the observations of the former were confined to the results of artificial section of spinal nerves, made for the purpose of studying the effects of such injuries, while those of Türck were of a purely pathological character, in which the results of old morbid deposits within the substance of the brain were studied by the aid of successive sections of the brain and spinal cord at different levels, which could be contrasted with each other.

Both of these observers arrived at the same fundamental law, viz., that injuries of nerves or of nerve-tracts which separated them from their centers of nutrition or trophic centers, cause a degenerative process which extends along the separate nerve-fibers to their ultimate ramifications. Waller's

The reader is referred to a lecture upon the "Wallerian Method of Research," by Prof. Dalton, "Med. Record," Feb. 11, 1882.

"Nerve-fibers degenerate only when severed from their connection with some special nerve-center, from which they receive their nutrition. These are called the "trophic centers" of the different bundles. When once cut off, the degenerative process extends throughout the entire length of the nerve; unless it meets another nerve-center (some ganglionic mass of gray substance) interposed in its course. It seldom, therefore, if ever, extends from spinal nerve-tracts into the spinal nerves, or vice versa.

experiments were confined exclusively to the spinal nerves, and resulted in the following deductions: 1, That if the nerve was divided at its exit from the vertebral canal, all of its ultimate fibers degenerated for its entire length; 2, that if the anterior root of the nerve was alone divided, only the motor fibers degenerated; 3, that if the posterior root of the nerve was severed outside of its ganglion, the sensory fibers of the nerve degenerated and the motor fibers remained unaffected; 4, that if the posterior root was divided internal to its ganglion, the nerve outside of the ganglion did not degenerate, but the portion which was still attached to the spinal cord, but separated from the ganglion, suffered complete degeneration. From these data, this observer was enabled to lay down the general law that the motor fibers of the spinal nerves are dependent for their structural integrity upon their connection with the spinal cord, while the sensory nerve-fibers depend upon their connection with the spinal ganglia.

The degenerative process which was recognized by both Türck and Waller consists in the segmentation of the myelin and the production of an excess of nuclei along the course of the affected nerve-fibers. The unaffected fibers retain their normal appearance, and thus define the diseased bundles so that they can be traced along the spinal cord and peripheral nerves with great accuracy.

Türck was enabled to demonstrate for the first time a distinction between the anterior and posterior segments of the lateral column of the spinal cord, which no ordinary dissection could possibly have established. The observations of Türck have been supplemented by those of Gudden, Goltz, Gull, Flechsig, Meynert, Rolando, Stilling, Foville, Gratiolet, Broadbent, Bourdon, Charcot, Spitzka, Starr, and others, who have added much to our knowledge of the situation and functions of the various spinal nerve-tracts.

Gudden's method of anatomical research consists in the destruction of nerve-tracts by operations performed upon newly-born animals. He found that, as a result of the injury done, the proximal end of the divided nerve atrophied, as

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well as the central connections of the nerve. The so-called Wallerian degeneration, of course, affects the distal portion of the nerve, simultaneously with the development of Gudden's degeneration of the proximal portion.

Flechsig's method consists in studying the relative period (during fetal development) at which certain nerve-strands acquire myelin.

Thus we have to-day the older methods (1) of actual dissection of nerve-bundles in partially hardened specimens, and (2) the comparison of a consecutive series of fine sections of the brain and spinal cord with each other, supplemented by the more accurate methods of Türck, Gudden, and Flechsig, as guides in our anatomical studies of the nervous system.

The study of microcephalic brains, although yet in its infancy, bears evidence of affording great possibilities in the future toward the elucidation of disputed points in cerebral and spinal anatomy. M. Allen Starr has lately reported a very interesting case of this character.

The fibers of the caudo-lenticular portion (Fig. 38) are probably deflected (in the pons) and pass to the cerebellum. The remaining fibers which lie anteriorly to the sensory tract are not so deflected.

Now, because motor fibers carry centrifugal impulses, it is logical to describe the motor bundles of the internal capsule from above downward, beginning with an enumeration of the convolutions from which the motor fibers are believed to spring, and tracing the course of these fibers to their connection with the cells of the anterior horns of the spinal gray matter, while it is customary to reverse the method, in case the sensory fibers, which carry centripetal impulses, are under consideration.

The diagram to which I shall first call your attention (Fig. 14) was designed by its author (Seguin) to rudely represent the general features of the internal capsule. It is impossible to properly portray all of the more important facts, to which I shall call attention, by any form of schematic drawing; so that the diagram offered, which is most excellent of its kind,