walls of the uterus, at the time of delivery, are composed of such muscular fibres, arranged in circular, oblique, and longitudinal bundles.

Fig. 279.

About the end of the first week after delivery, these fibres begin to undergo a fatty degeneration. (Fig. 278.) Their granules become larger and more prominent, and soon assume the appearance of fat granules, deposited in the substance of the fibre. The deposit, thus commenced, increases in abundance, and the granules continue to enlarge until they become converted into fully formed fat globules, which fill the interior of the fibre more or less completely, and mask, to a certain extent, its anatomical characters. (Fig. 279.) The fatty degeneration, thus induced, gives to the uterus a softer consistency, and a pale yellowish color which is characteristic of this period. The altered muscular fibres are afterward absorbed, and gradually give place to others of new forma

MUSCULAR FIBRES OF HUMAN UTERUS

of peritonitis.

three weeks after parturition; from a woman dead tion, which already begin to show themselves before the old ones have disappeared.

The process finally results 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 unimpregnated condition. The entire renewal or reconstruction of the uterus is completed, according to Heschl, about the end of the second month after delivery

CHAPTER XIV.

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

THE first trace of the cerebro-spinal axis in the embryo consists of the two longitudinal folds of the external blastodermic layer, which include between them the median furrow, known as the "medullary groove" (page 724). The two folds, after uniting by their corresponding edges on the median line, over the back of the embryo, convert the groove into a canal, the "medullary canal;" and it is within this canal that the cerebrospinal axis is formed.

The mode of its formation is by the growth of nervous matter upon the inner surface of the medullary canal; and this canal, which becomes the cerebrospinal canal, is accordingly lined with a secondary internal sheath of nervous matter, which also has the form of a tubular membranous canal, with a continuous central cavity. This is the cerebro-spinal axis, which thus forms a hollow cylindrical cord of nervous matter, running in a longitudinal direction. within the cerebro-spinal canal. Anteriorly it expands into a bulbous enlargement corresponding to the brain. Its middle portion, constituting the spinal cord, is nearly cylindrical; and posteriorly, at its caudal extremity, it terminates by a pointed enlargement.

Formation of the 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, partially separated from each other by incomplete transverse constrictions. These are known as the cerebral vesicles, from which the different parts of the encephalon are afterward to be developed. The first or most anterior vesicle is destined to form the hemispheres; the second or middle, the tubercula quadrigemina; the third, or posterior, the medulla oblongata. All three vesicles are still hollow, and their cavities communicate freely with each other through the intervening orifices.

Very soon the anterior and posterior cerebral vesicles undergo 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, the posterior the medulla oblongata. Fig. 281.

Fig. 282.

Formation of the CEREBRO-SPINAL AX18.-1. Vesicle of the hemispheres. 2. Vesicle of the tubercula quadrigemina. 3. Vesicle of the medulla oblongata.

FETAL PIG, 1 centimetre long, showing the condition of the brain and spinal cord.-1. Hemispheres. 2. Tubercula quadrigemina. 3. Cerebellum. 4. Medulla oblongata.

There are, therefore, at this time five cerebral vesicles, all of which communicate with each other and with the central cavity of the spinal cord. The entire cerebro-spinal axis also becomes strongly curved in an anterior direction, corresponding with the anterior curvature of the body of the embryo (Fig. 282); 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. At first the relative size of the various parts of the encephalon is very different from that presented in the adult condition. The hemispheres are hardly larger than the tubercula quadrigemina; and the cerebellum is inferior in size to the medulla oblongata. Soon afterward, the relative position and volume of the parts begin to alter. The hemispheres and tubercula quadrigemina grow faster than the posterior portions of the encephalon; and the cerebellum becomes doubled backward over the medulla oblongata. (Fig. 283.) SubseFig. 283.

Fig. 284.

quently, the hemispheres enlarge more rapidly, growing upward and backward, so as to cover both the optic thalami and the tubercula

quadrigemina (Fig. 284); the cerebellum tending in the same way to grow backward, and projecting farther in this direction over the medulla oblongata. The subsequent history of the development of the encephalon is mainly a continuation of the same process; the relative dimensions of the parts constantly changing, so that the hemispheres become, in the adult condition (Fig. 285), the largest division of the encephalon,

BRAIN OF ADULT PIG.-1. Hemispheres. 3. Cerebellum. 4. Medulla oblongata. while the cerebellum is next in size, and covers the upper portion of the medulla oblongata. The surfaces of the hemispheres and cerebellum, which are at first smooth, become afterward convoluted; thus increasing still farther the extent of their nervous matter. In the human fœtus the cerebral convolutions begin to appear about the beginning of the fifth month (Longet), and grow deeper and more abundant during the remainder of fœtal life.

The lateral portions of the brain growing at the same time more rapidly than that on the median line, they project on each side outward and upward; and by folding over against each other toward the median line, they form the right and left hemispheres, separated by the longitudinal fissure. A similar process of growth in the spinal cord results in the formation of its two lateral halves, and the anterior and posterior median fissures of the cord. Elsewhere the median fissure is less com. plete, as, for example, between the two lateral halves of the cerebellum or those of the medulla oblongata; but it exists everywhere, and marks more or less distinctly the division between the two sides of the nervous centres, produced by the excessive growth of their lateral portions. In this way the whole cerebro-spinal axis is converted into a double organ, equally developed upon the right and left sides, and partially divided by longitudinal median fissures.

Organs of Special Sense.-The eyes are formed by a diverticulum which grows out on each side from the first cerebral vesicle. This diverticulum is at first hollow, its cavity communicating with that of the hemisphere. Afterward, the passage between the two is filled with a deposit of nervous matter, and becomes the optic nerve. The globular portion of the diverticulum, which is converted into the globe of the eye, has a thin layer of nervous matter deposited upon its internal sur

face, which becomes the retina; the rest of its cavity being occupied by a gelatinous substance, the vitreous body. The crystalline lens is formed in a distinct follicle, which is an offshoot of the integument, and becomes partially imbedded in the anterior portion of the eyeball. The cornea also is originally a part of the integument, and remains somewhat opaque until a late period of development. It becomes nearly transparent a short time before birth.

The iris is a muscular septum, formed in front of the crystalline lens. Its central opening, which afterward becomes the pupil, is at first closed by a vascular membrane, the pupillary membrane, passing across the axis of the eye. The blood vessels of this membrane, which are derived from those of the iris, subsequently become atrophied. They disappear first from its centre, and recede gradually toward its circumference; returning upon themselves in loops, the convexities of which are directed toward the centre. The pupillary membrane itself finally becomes atrophied, following in this retrograde process the direction of its receding blood vessels, namely, from the centre outward. It has completely disappeared by the end of the seventh month. (Cruveilhier.)

The eyelids are formed by folds of the integument, which project from above and below at the situation of the eyeball. They grow so rapidly during the second and third months that their free margins come in contact and adhere together, so that at that time they cannot be separated without some violence. They remain adherent from this period until the seventh month (Guy), when their margins separate and they become free and movable. In carnivorous animals (dogs and cats), the eyelids do not separate from each other until eight or ten days after birth.

The internal ear is formed in a somewhat similar manner with the eyeball, by an offshoot from the third cerebral vesicle; the passage between them filling up by a deposit of white substance, which becomes the auditory nerve. The tympanum and auditory meatus are both offshoots from the external integument.

Skeleton and Limbs.-At a very early period of development there appears, immediately beneath the medullary canal, a cylindrical cord, termed the chorda dorsalis (page 725). It consists of a tubular sheath containing a mass of simple cells, closely packed together and united by adhesive material. It does not become a permanent part of the skeleton, but is a temporary organ destined to disappear as development proceeds.

On each side of the chorda dorsalis there is formed a series of rectangular plates, the "primitive vertebræ," a portion of whose substance is devoted to the formation of muscular tissue, while another portion becomes the basis for the permanent vertebræ. The latter are deposited in the form of cartilaginous plates, which encircle the chorda dorsalis in a series of rings, corresponding in number with the bodies of the future vertebræ. The rings increase in thickness from without inward, encroaching upon the substance of the chorda dorsalis, and