impulses to the muscles; but that there are many groups of nerve cells which can take on the duty of the injured cells and act for them in receiving sensory and discharging motor impulses.

It has already been pointed out that the function of any given nerve fibre depends on the relationship of its terminals. fibre itself is merely a conducting agent. In somewhat the same way the functions of any given nerve cell must depend on the

The shading shows the great extent of surface destroyed before permanent cortical blindness followed by retinal atrophy was produced; i. e., all posterior lobes and angular gyri.

number and character of its connections. If it be connected with a motorial end plate in a muscle, it can only excite impulses that give rise to motion: if it be connected with a sensory terminal, it can only be a receiver of sensory impulses. But, in the gray matter of the spinal cord, and still more so in that of the cerebral cortex, we may assume that all the cells are in more or less intimate connection with innumerable other cells. In fact,

we must imagine that the gray matter of both cord and brain is interwoven into a complex texture of fibrils and cells, no part of which is isolated from the rest, but all the elements form part of a continuous system, and within certain limits can subsidize each other's functions.

When we excise, cauterize or stimulate a given point of the complex cortex, we do not know in what way we interfere with the perfect action of that wonderful nervous nexus which controls the organism, for we can only judge of the effects we produce by results limited to those few functions the activity of which is obvious.

CHAPTER XXXVII.

REPRODUCTION.

MALE AND FEMALE GENERATIVE ELEMENTS.

One of the chief characteristics of living beings is their power of reproduction; that is to say, organisms can, under favorable conditions, form other individuals with lives and habits similar to their own.

In the lowest forms of animal life this propagation of species may take place by the division of a single cell: thus an amoeba reproduces by the cleavage of its mass of protoplasm, separating the main body into two amoeba. Such a method of reproduction is purely asexual, each individual having the intrinsic power of reproduction.

As we ascend the animal scale, we find that, just as other functions are executed by certain specially differentiated groups of cells, so reproduction is performed by certain collections of cells endowed with specific powers. Further, we find that the production of a new being requires the coöperation of two kinds of generative elements, each of which is produced by a distinct organ. In the higher organisms these reproductive elements are produced by different individuals of the same species, thereby dividing them into two sexes. This is termed sexual reproduction.

The sexual method of reproduction is met with in all the more highly developed forms of animal and vegetable life. The male organ produces active elements-the spermatozoa; the female organ produces the ovum, which, when fertilized by the spermatozoa, develops embryo.

In mammalia the uterus is a most important subsidiary organ, as it becomes modified to allow of the development and growth of the embryo; its earlier functions, however, can be performed by other organs, as seen in cases of extra-uterine foetation, when the ovum develops in some unusual situation, such as the Fallopian tube or the abdominal cavity.

The spermatozoa are formed indirectly from the cells lining the tubuli seminiferi of the testicle. These cells, cubical masses of protoplasm, give rise to others (spermatoblasts), which form another layer and undergo rapid proliferation. The nuclei divide, and. from each part arises the head of a spermatozoön, the body being developed from the protoplasm of the cell. The spermatic elements escape into the tubes, and pass down the vasa deferentia into the vesicula seminales, where they either undergo retrograde change or are cast out of the body.

Section of the tubuli seminiferi of a rat. (Schäfer.)

a, Tubuli in which the spermatozoa are not fully developed. 6. Spermatozoa more developed. c. Spermatozoa fully developed.

The ovum arises from the differentiation of a cell from the germ epithelium covering the surface of the ovary. A group of these cells entering the periphery of the ovary, becomes there embedded in a kind of capsule derived from the surrounding areolar tissue of the stroma, and forms an immature Graafian follicle. A central cell grows rapidly to form the ovum, the rest increase in number to form the small cells of the granular tunic. As the follicle develops, it works its way toward the centre of the ovary,

and subsequently approaches the periphery of the organ as a fully-developed Graafian follicle.

Microscopically, it is seen to be surrounded by a capsule, tunica fibrosa, which is ill-defined from the stroma of the ovary

Section of the ovary of a cat, showing the origin and the development of Graafian follicles.

a. Germ epithelium.

b. Graafian follicle partly developed. c. Earliest form of Graafian follicle.

d. Well-developed Graafian follicle.

(Cadiat.)

e. Ovum.

in which it lies. Outside this is a layer of capillary blood vessels, tunica vasculosa, and to these two coats collectively the term tunica propria is applied.