Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/426

408 KEPKODUCTION [ANIMAL. In many forms it is not easy to see bow the ova once liberated into the body-cavity find their way safely into the small opening of the discontinuous oviduct. In the Frog, however, tracts of the peritoneal epithelium become ciliated, so propelling the ova in the right direction. In Reptiles, Birds, and Mammals the open end of the oviduct is widened and fringed, and lies close to or even touching the ovary ; muscular fibres too are present, and more or less active movements of this dilated end over the ovarian surface have been alleged to occur. The oviduct once reached, the downward progress of the ovum is ensured by the cilia of the epithelial lining, and probably also by peristaltic movements of its muscular coat. (I) 1 Menstruation. The process of menstruation (menses, catamenia), although from the earliest times the subject of medical inquiry, is by no means yet clearly understood. It occurs usually at intervals of a lunar month in all women during their period of potential fertility (fecundity), and, so far from being confined to the human species, has been observed at the period of "heat" in a large number of Mammals. Though thus clearly a normal physiological process, it yet evidently lies on the borders of pathological change, as is evidenced not only by the pain which so fre- quently accompanies it, and the local and constitutional disorders which so frequently arise in this connexion, but by the general systemic disturbance and local histological changes of which the discharge is merely the outward expression and result. The histological facts are briefly as follows. The mucous lining of the uterus consists of a loose vascular connective tissue covered by ciliated epithe- lium and containing numerous glands of clear alkaline secretion. This mucous lining before the outset of men- struation becomes loose and cedematous, its lymphatics being greatly distended ; it thus thickens considerably, pressing against the cervix of the uterus. An extrava- sation of blood from the capillaries next takes place over the whole surface of the mucous layer, and the discharge is thus set up. This consists at first of blood largely diluted with the secretion of the uterine glands, but soon becomes mixed with detritus from the disintegration of the mucous coat, of which not only the general epithelial cells but those of the neck of the glands, and even part of the subjacent connective tissue, undergo fatty degeneration and fall off, occasionally even in a mass. After from three to six days the blood ceases to appear, and the lost epithe- lium is rapidly replaced, apparently by proliferation from the necks of the glands. By the ninth or tenth day the mucous coat is fully healed and the beginnings of the next menstrual process recommence. The age at which the process commences varies with race and climate, with nutrition and growth, with habit of life (e.g., with differences between town and country life), and with mental and moral characteristics. Of these, however, climate seems most important ; thus, while in northern Europe the average age is reckoned at the begin- ning of the fifteenth year, in the tropics it seems to commence in the ninth or tenth. The cessation of men- struation usually takes place between the age of forty-five and fifty, and, somewhat as the secondary characteristics of female puberty coincide with its appearance, a less distinct reduction of these is associated with its close ; in many cases secondary resemblances to the masculine type may supervene. The old theories of menstruation were that it served to rid the system of impure blood, that it simply corre- sponded to the period of " heat " observed in lower animals, or, later, that it was associated with ovulation, which indeed seems broadly to correspond with the end of 1 These numbers refer to the bibliography at p. 422. the menstrual period. At present there may be said to be two rival theories. According to the first of these the pro- cess is viewed as a kind of surgical "freshening "of the uterus for the reception of the ovum, whereby the latter during the healing process can be attached safely to the uterine wall. The other view is exactly the reverse of this. Its upholders regard the growth of the mucous coat before the commencement of the flow as a preparation for the reception of an ovum, if duly fertilized, and the menstrual process itself as the expression of the failure of these pre- parations, in short as a consequence of the non-occurrence of pregnancy. A decided majority of gynaecologists appear to incline to the latter view. (1) Comparative Anatomy of the Reproductive Organs. The multicellular animals afford a remarkable series of grada- tions from the simplest imaginable case in which certain cells, independently of accessory organs, and even isolated from each other, develop into ova and spermatozoa. In the vast majority of cases, however, definite groups of cells are set apart as the essential reproductive glands the ovary and the testis. The contents of these may simply break loose, but definite excretory ducts are very frequently present, and upon these very varied complications may arise. To the male ducts a seminal reservoir may be added. More or less specialized glandular regions may contribute their secretion to the seminal fluid, and a more or less complicated copulatory apparatus may also be superadded. The female accessory organs are equally simple in prin- ciple and complex in detail. Nutritive material may be furnished to the ova by special yolk glands, or by the walls of the oviduct ; this too may supply special enve- lopes, and may exhibit dilatations for the preservation or development of the ova (uterus), for the reception of the male copulatory organ (vagina), or for the temporary storage of the seminal fluid thus introduced (receptaculum seminis). It is necessary therefore briefly to outline the most important facts of the comparative anatomy and physiology of these organs in the various groups. Passing over the little-understood Orthonectida and Dicyemida (see PARASITISM, vol. xviii. p. 259), the Sponges present the very simplest case above referred to. Here and there throughout the mesoderm a cell may be observed enlarging to form an ovum, or segmenting to form a mass of spermatozoa, but no definite reproductive glands, much less any duct or accessory organ, are present, and at most the ovum forms for itself a kind of nest among the sur- rounding cells, an approach towards the epithelial follicle of higher forms being thus presented. Ccelentera. In Anthozoa the generative organs are developed as ridges on the gastric septa, their products passing out by the mouth ; and in the Ctenophora each radial canal bears an ovarian ridge on one side and a testicular on the other. Keen controversy has raged over the state of matters in the Hydromedusx. Kleinenberg derived both ovum and sperms in Hydra from the ecto- derm, while Van Beneden endeavoured to prove the invariable origin of the ovarian tract from the endoderm and that of the testicular from the ectoderm ; but sub- sequent observers, so far from confirming the constancy of this arrangement, have affirmed in many forms the ecto- dermic and in others the endodermic origin in both cases. Weismann and De Varenne have recently very completely demonstrated the more remarkable fact that in a great number of forms the generative elements do not arise in the so-called generative buds or gonophores at all, but actually migrate thither from the parenchyma (cceuen- chyma) of the nutritive polyp or trophosome the latter observer indeed going so far as to allege the primitive origin by ova and spermatozoa in all cases from the endo-