ments were sterilized in boiling water; the site of the wound was freed from hair and washed in mercuric chloride, 1-1000. After the operation the opening was sealed with collodion.

It was not until many failures had been made that success was attained in raising the rats operated on during the first week of life. In the first cases when they were returned to the nest the mother ate them, and when fed by hand they could be kept alive for only a short time. Others working in this laboratory have experienced the same difficulty. It was supposed that the action of the mother was caused by some foreign odor clinging to the young rat after it had been handled. A certain degree of success, however, was secured by keeping the young in an incubator heated nearly to the body temperature, and forcing the mother, kept in a cage near by, to nurse them at regular intervals (every four hours during the day and eight hours at night). By this means some young were kept alive for ten days. It was found that as long as the young were kept warm the mother could safely be left with them and that after a time she would care for them of her own accord. It was also found that if the animals were taken from the nest under conditions which prevented the loss of body-heat, they could be returned to the mother with safety. For this purpose a jar was warmed to body temperature and filled with hot cotton. In this the rat was carried from the nest to the opera. ting room, where it was placed in an incubator until all was ready. The rat was held in a hot cloth during the operation, which was performed in a few minutes and without anaesthetics. It was then returned to the hot jar and carried back to the animal room. When these precautions were observed the mother never refused to care for the young.

After the operation the rats were allowed to live for one month and a half. The brains were then removed, hardened in MÜLLER'S fluid at 40° C. for one month and imbedded in celloidin. The occipital region of each brain was cut in serial frontal sections 45 μ thick and stained by the PAL-WEIGERT technique. Also for comparison the brain of a rat, operated on at the age of four days and killed thirty-two days later, was

hardened in VAN GEHUCHTEN's solution, imbedded in paraffin, cut in serial sections 6 thick, and stained in erythrosin and toluidin blue.

Results.

It will be most convenient to give a general description of the wounds in each of the four animals, and to follow this with a summary of the more important points as they are seen when the four specimens are compared. It will be found that this description differs from the usual accounts in the following points: (1) the absence of adhesions in the meninges, (2) the absence of a well defined connective tissue scar, (3) the atrophy of sectioned nerve fibers on the cell body side of the injury, (4) the distortion of the wound due to the shifting of areas in the growing cortex and (5) the presence of nerve fibers crossing the site of the lesion.

Rat No. I was operated on when twenty-one days old and was killed fifty-three days later. The wound was found in the occipital region of the left hemisphere, parallel to, and near the middle line. On examining the serial sections of this region the wound could be traced from near the posterior edge of the splenium forward for a little over 2 mm. In its anterior part it penetrates the cornu Ammonis and runs a slight distance into the thalamus. From this point the floor of the wound gradually slopes upward and backward until the surface of the cortex is reached. By reference to Fig. 1 the general character of the wound will be seen. It is visible as a light band running down from the surface of the cortex and extending just through the corpus callosum, but it does not at this point extend into the cornu Ammonis. Throughout its whole extent dark granular masses are apparent-the remains of unabsorbed blood. The scar tissue is quite abundant in this brain clearly marking off the wound from the normal brain substance. On comparing this preparation with a corresponding one in a set of serial sections of a normal rat brain, it was found that the banking up of fibers on the medial side of the wound is not entirely the result of the operation, as an evident excess of fibers on the

medial side is to be seen at a corresponding level in the normal brain. It happened that the knife passed along the line that forms the lateral limit of the medial area of dense fibers, and the presence of a band of scar tissue almost devoid of fibers enhances the contrast to some extent.

The light band which marks the path of the knife is almost devoid of fibers; but a few isolated ones may be distinguished in the scar. Although clearly visible in the original preparation, they do not appear in the photograph which was taken with a very low power. In order to show these more accurately than could be done by a drawing on a small scale, a camera lucida tracing has been made (Fig. 5.). This tracing was taken from the same preparation that was used for the photograph, and represents a point in the lesion one-third of the way from the corpus callosum to the surface of the cortex. This preparation is No. 27 in a series of 45 passing through this wound; it is therefore at some distance from either the anterior or posterior extremity of the lesion. These new formed fibers follow a very irregular course, so that it is not possible to trace them far in the plane of any one section. But in a number of cases it is possible to follow single fibers from the normal brain substance on one side through the scar tissue and into the normal brain substance on the other side. Such a fiber is seen in Fig. 5, f.

Thus two tests were rigorously applied before the conclusion was formed that the fibers in the scar were really new elements; 1st, The fiber in question must pass completely across the lesion, making connections with the brain tissue on both sides; and 2nd, The fiber must cross near the geometric center of the lesion. Fibers that passed into the scar and then back again into the brain substance on the side from which they emerged were not counted as new-formed, for these might have been displaced into the opening left by the knife. One could not be sure that they had been formed since the operation. Fibers that passed across the lesion near its anterior or posterior extremities or near its lower limit were also disregarded; for these might have been uninjured fibers crowded

into the degenerated areas. But a fiber such as is illustrated in the tracing, crossing the wound near its center and making connections with the brain tissue on both sides of the scar, must without doubt be new-formed.

Rat No. 2 was operated on when seven days old and killed forty-two days later. The wound in the skull was indicated only by a slight roughness, ossification at this point being as complete as elsewhere. The dura mater was normal and without adhesions to the scar in the skull or brain. The brain scar is not visible, its locality being indicated only by the arrangement of the surface veins which encircle it.

In serial sections the lesion is clearly defined for a distance of 1.5 mm., of which 1.2 mm. lies posterior to the line of the splenium. Throughout its whole extent the wound passes through the substantia alba of the occipital lobe, into the cortex covering its under surface. Scattered along through the white matter there is a considerable amount of unabsorbed blood pigment which was, however, largely absent from the section of which Fig. 2 is a photograph. As will be seen by reference to that figure, the cortex on the upper surface of the hemisphere has healed so completely that it is hardly possible to make out the path of the knife in the upper two-thirds of the grey matter, while in its lower third and in the substantia alba the lesion is clearly visible. Although the distribution of the fibers in the upper region of the cortex is quite normal, the knife must have passed through this area to have reached the structures below.

Somewhat lateral ward to the wound in the upper part of the hemisphere is a peculiar aggregation of fibers in the cortex covering the ventral surface. This region is shown in more detail in Fig. 9. The dark granular masses indicate the line along which the knife has passed. Numerous fibers running downward from the white matter are seen to follow this line and on approaching the ventral surface to spread out in a fanshaped area. Nerve fibers in considerable numbers may be seen crossing the site of the lesion. Especially is this noticeable just below the substantia alba (ƒ).

It will be seen (Fig. 2) that the lines indicating the path of the knife above and below, the white matter are not continuous, that below being more lateralward than that above, and running at a different angle. Since the injury was produced by a straight stab, this appearance can be explained only on the supposition that the areas of the cortex have shifted their relative positions during the process of growth. The shifting that takes place in the cortex over the corpus callosum is just the reverse, as seen in the sections near the anterior extremity of this same wound, where the part above the corpus callosum is more lateralward than the part below.

Rat No. 3 was operated on when three days of age and killed forty-one days later. No sign of the injury could be found in the skull or the dura mater. On the brain a pale pinkish groove, 1.5 mm. in length, marked the line along which the knife had entered the cortex.

When serial sections are examined, it is observed that the healing process is more complete than in the brains of the two older rats. The remains of unabsorbed blood are scarcely to be found; and at its anterior and posterior extremities the wound fades off so gradually into the normal brain substance that it is impossible by a study of serial sections to determine its exact limits. Its posterior extremity is slightly behind the end of the splenium, from which point it runs forward for a distance of a little more than 1.5 mm. By reference to Fig. 3 the general appearance of the wound will be seen. Lateral to the point of the lesion the corpus callosum has entirely disappeared from the injured region. A small portion of its fibers remains intact near the middle line; but elsewhere the substantia alba of the cortex rests immediately on the cornu Ammonis and the alveus. The alveus is absent for a short distance. The point near the middle line where the corpus callosum and alveus end represents the original position of the wound. By the large growth of fibers in the cortex medial to the lesion, the scar has been shifted lateralward until it has assumed the shape of a dilated V.

The disappearance of the corpus callosum lateralward to