the injury is of special interest. Had the sectioned fibers of the callosum merely degenerated in the part distal to the lesion, there would have been a considerable number of fibers in the corpus callosum on each side of the injury, instead of an entire absence of them on the lateral side. Their absence on one side and presence on the other can be understood by supposing that the fibers cut near their cells of origin-fibers whose cells were situated in the injured hemisphere-degenerated throughout their whole extent, while fibers cut at a greater distance from their cells of origin— fibers whose cells were situated in the opposite hemispheredegenerated upon the distal side of the lesion only. In VON GUDDEN'S Gesammelte Abhandlungen (34) there is recorded a similar observation. Six or eight weeks after removal of one cerebral hemisphere in a young rabbit, he found that the anterior commissure and corpus callosum had completely atrophied in the remaining hemisphere.

In the upper region of the cortex the wound is recognized with difficulty, because there is so little scar tissue and the number of crossing fibers is so great. It can, however, be seen as a faint line along the path the knife must have taken to reach the underlying parts where the injury is more apparent. Fig. 6 is a camera lucida tracing of some of the fibers in the scar. It was taken from the same preparation as Fig. 3 about one-third the way up from the corpus callosum to the cortex. Two fibers fulfil the requirement of passing into the brain tissue on each side, and the others would probably do so if they could be followed far enough.

Rat No. 4 was twelve hours old at the time of the operation and forty days old when killed. The meninges were quite normal, and there was no sign of the injury apparent on the surface of the brain. Serial sections revealed a lesion extending 1 mm. behind the posterior extremity of the splenium and almost 1⁄2 mm. in front of that point. The general appearance of the wound is shown in Fig. 4. The upper part of the cortex is normal, no scar tissue can be seen there, and nerve fibers have a normal distribution. In the lower half is a trian

gular area lighter than the rest, representing scar tissue that has taken the place of a large blood clot. This runs through all the sections, giving, when reconstructed, a mass in the shape of a three-sided prism with base directed ventrally. Below this lies the corpus callosum degenerated on both sides of the lesion. In this case the axons degenerated toward the cell whether cut before or after crossing the middle line. few straggling fibers are seen in the otherwise unstained area. These probably represent neurones which have completed their development since the operation.

In the tissue composing the scar numerous nerve fibers pass in every direction interlacing to form a large meshed irregular network (Fig. 7). Fig. 8 is a camera lucida tracing of a part of this same area from another section of this brain. Owing to the unusual width of the scar in this region only a part of it could be represented in the tracing; the brain tissue along its margins is not indicated as in previous tracings.

The presence of medullated nerve fibers in the cicatricial tissue, demonstrated in each of the four brains, might be explained on either of two hypotheses. They might represent regenerated portions of nerve fibers cut by the operation; or they might have been formed since the operation by outgrowths from cell bodies incompletely developed at that time. Although there is nothing in the appearance of any given section to exclude the former theory, the latter seems the more probable. This conclusion is supported by the uniformly negative results obtained by those who have studied wounds in the brains of adult mammals; and by the fact that in the four cases given here the number of fibers in the scar diminishes regularly as the age of the animal at the time of the operation increases. (Compare Fig. 8 with Figs. 6 and 5). These facts might be explained as indicating a loss of the regenerative capacity of the neurones within the brain as the animal becomes older; but are more probably due to the diminution in the number of undeveloped cells capable of sending axons across the lesion subsequently to the injury. During the period of active growth with which we have to deal the diminution in the number of

these cells would be sufficiently rapid to explain the decrease in the number of the crossing fibers. An observation, which tells strongly against the regeneration theory in this case, is that the fibers of the corpus callosum degenerated toward their cells of origin after being cut in the two younger rats, while they remained intact on the cell side of the injury in older rats. This observation shows that nerve fibers instead of possessing unusual regenerative power in these young animals (where the largest number of nerve fibers were found in the scar) tend to degenerate completely after injury.

Summary.

(1). The adhesions which are described by other investigators as binding the brain scar to the meninges and the meninges to the tissue filling the skull wound were entirely absent in these young rats.

(2). Very little scar tissue was found in the brain. In the upper part of the cortex it was not noticeable in PALWEIGERT preparations; in the substantia alba it was more abundant. With the exception of the large triangular area in the brain of rat No. 4, there is a steady decrease in the amount of scar tissue with the decrease in the age of the animal at the time of the operation. How little connective tissue there is in the wound is seen in Fig. 10. Nerve cells quite normal in appearance border directly on the line that represents the path of the knife. Only here and there can a connective tissue cell be found in the scar (c). In the brains of older animals the connective tissue is very abundant at the site of the lesion. It may be that the absence of a dense connective tissue scar in the cases here given may in part explain the positive results obtained on the crossing of nerve fibers, but the increase in the number of nerve fibers is out of proportion to the decrease in the amount of scar tissue.

(3). A degeneration of the fibers of the corpus callosum toward their cells of origin was noticed in the two younger

rats.

(4). Shifting of the parts of the cortex with reference to

each other occurred in all but the oldest rat. In rats Nos. 2, 3 and 4 it was found that the parts of the cortex had altered their relative positions in the same manner, such that if we consider the under surface of the cortex as stationery the cortex upon the upper surface shifts lateralward over the splenium, Fig. 3, and medialward in the posterior part of the occipital lobe, Fig. 2. This is explained by the fact that when a large number of fibers develops in any area, that area increases in size more rapidly than the surrounding parts. In Fig. 3 is seen a dense area of fibers just medial to the scar, which is responsible for the lateral displacement of the scar.

(5). We have seen that at the center of stab wounds, where every fiber must have been cut by the knife, medullated nerve fibers may be traced across the wound and into the normal brain tissue on each side. While there can be no doubt that these are new formed elements, they represent in all prob. ability not regenerated but entirely new axons. The number of such fibers is very great in the youngest rats, but decreases quite rapidly as the age of the rat at the time of the operation increases. Unfortunately the brains of rats operated on at a more advanced age were spoiled in the process of hardening, and I cannot say at what time the brain of the rat ceases to have the capacity of sending fibers across the site of the lesion.

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