this spot afterward cease to be visible. At the point, however, where the placenta was attached, the regeneration of the mucous membrane is less rapid; and a cicatrix-like spot is often visible at this situation for several months after delivery.

The only further change, which remains to be described in this connection, is the fatty degeneration and reconstruction of the muscular substance of the uterus. This process, which is sometimes known as the "involu

tion" of the uterus, takes place in the following manner. The muscular fibres of the unimpregnated uterus are pale, flattened, spindleshaped bodies (Fig.214) nearly homogeneous in structure or very faintly granular, and measuring from ʊ to zʊ of an inch in length, by Toooo to Gooo of an inch in width. During gestation these fibres increase very considerably in size. Their texture becomes much more distinctly granular, and their outlines more strongly marked. An oval nucleus also shows itself in the central part of each fibre. The entire walls of the uterus, at the time of delivery, are composed of such muscular fibres as these, arranged in circular, oblique, and longitudinal bundles.

Fig. 214.

MUSCULAR FIBRES OF UNIMPREGNATED UTERUS; from a woman aged 40, dead of phthisis pulmonalis.

About the end of the first week after delivery, these fibres begin to undergo a fatty degeneration. (Fig. 215.) Their granules become larger and more prominent, and very soon assume the appearance of peral fever.

Fig. 215.

MUSCULAR FIBRES OF HUMAN UTERUS, ten days after parturition; from a woman dead of puer

molecules of fat, deposited in the substance of the fibre. The fatty deposit, thus commenced, increases in abundance, and the molecules continue to enlarge until they become converted into fully formed oil-globules, which fill the interior of the fibre more or less

completely, and mask, to a certain extent, its anatomical characters. (Fig. 216.) The universal fatty degeneration, thus induced, gives to the uterus a softer consistency, and a pale yellowish color which is characteristic of it at this period. The muscu

lar fibres which have become altered by the fatty deposit are afterward gradually absorbed and disappear; their place being subsequently taken by other fibres of new

MUSCULAR FIBRES OF HUMAN UTERUS, three formation, which already be

weeks after parturition; from a woman dead of peritonitis.

gin to make their appearance before the old ones have been

completely destroyed. As this process goes on, it results finally in a complete renovation of the muscular substance of the uterus. The organ becomes again reduced in size, compact in tissue, and of a pale ruddy hue, as in the ordinary unimpregnated condition. This entire renewal or reconstruction of the uterus is completed, according to Heschl,' about the end of the second month after delivery.

1 Op. cit.

CHAPTER XIV.

DEVELOPMENT OF THE EMBRYO-NERVOUS SYSTEM, ORGANS OF SENSE, SKELETON, AND LIMBS.

THE first trace of a spinal cord in the embryo consists of the double longitudinal fold or ridge of the blastodermic membrane, which shows itself at an early period, as above described, on each side the median furrow. The two lamina of which this is composed, on the right and left sides (Fig. 217, a, b), unite with each other in front, forming a rounded dilatation (c), the cephalic extremity, and behind at d, forming a pointed or caudal extremity. Near the posterior extremity, there is a smaller dilatation, which marks the future situation of the lumbar enlargement of the spinal cord.

As the laminæ above described grow upward and backward, they unite with each other upon the median line, so that the whole is converted into a hollow cylindrical cord, terminating anteriorly by a bulbous enlargement, and posteriorly by a pointed enlargement; the central cavity which it contains running continuously through it, from front to rear.

Fig. 217.

Formation of CEREBRO-SPINAL AX18.

a, b. Spinal cord.

c. Cephalic extremity. d.

Caudal extremity.

The next change which shows itself is a division of the anterior bulbous enlargement into three secondary compartments or vesicles (Fig. 218), which are partially separated from each other by transverse constrictions. These vesicles are known as the three cerebral vesicles, from which all the different parts of the encephalon are afterward to be developed. The first, or most anterior cerebral vesicle is destined to form the hemispheres; the second, or middle, the tubercula quadrigemina; and the third, or posterior, the medulla oblongata. All three vesicles are at this time hollow, and their

cavities communicate freely with each other, through the intervening constrictions.

Very soon the anterior and the posterior cerebral vesicles suffer

Fig. 218.

a further division; the middle one remaining undivided. The anterior vesicle thus separates into two portions, of which the first, or larger, constitutes the hemispheres, while the second, or smaller, becomes the optic thalami. The third vesicle also separates into two portions, of which the anterior becomes the cerebellum, and the posterior the medulla oblongata.

There are, therefore, at this time, five cerebral vesicles, all of whose cavities communicate with each other and with the central cavity of the spinal cord. The entire cerebro-spinal axis, at the same time, becomes very strongly curved in an anterior direction, corresponding with the anterior curvature of the body of the embryo (Fig. 219); so that the middle vesicle, or that of the tubercula quadrigemina, occupies a prominent angle at the upper part of the encephalon, while the hemispheres and the medulla oblongata are situated below it, anteriorly and posteriorly.

the hemispheres. 2. Vesicle of Vesicle of the medulla oblongata.

the tubercula quadrigemina. 3.

Fig. 219.

2 3

eighths of an inch

At first, it will be observed, the relative size of the various parts of the encephalon is very different from that which they afterward attain in the adult condition. The hemispheres, for example, are hardly larger than the tubercula quadrigemina; and the cerebellum is very much inferior in size to the medulla oblongata. Soon afterward, the relative position and size FETAL PIG, five of the parts begin to alter. The hemispheres and long, showing brain tubercula quadrigemina grow faster than the posteand spinal cord.-1. rior portions of the encephalon; and the cerebellum bercula quadrigemi- becomes doubled backward over the medulla oblon4. Medulla oblongata. gata. (Fig. 220.) Subsequently, the hemispheres rapidly enlarge, growing upward and backward, so as to cover in and conceal both the optic thalami and the tubercula quadrigemina (Fig. 221); the cerebellum tending in the same way to grow backward, and projecting farther and farther over the

Hemispheres. 2. Tu

na. 3. Cerebellum.

medulla oblongata. The subsequent history of the development of the encephalon is little more than a continuation of the same

process; the relative dimensions of the parts constantly changing, so that the hemispheres become, in the adult condition (Fig. 222),

the largest of all the divisions of the encephalon, while the cerebellum is next in size, and covers entirely the upper portion of the medulla oblongata. The surfaces, also, of the hemispheres and cerebellum, which were at first smooth, become afterward convoluted; increasing, in this way, still farther the extent of their nervous matter. In the human foetus, these convolutions begin to appear about the beginning of the fifth month (Longet), and grow constantly deeper and more abundant during the remainder of foetal life.

The lateral portions of the brain growing at the same time more rapidly than that which is situated on the median line, they soon project on each side outward and upward; and, by folding over against each other in the median line, form the right and left hemispheres, separated from each other by the longitudinal fissure.