After the corpus luteum has attained its full magnitude, its colour becomes paler and of a clearer yellow; its size then gradually dimi nishes, its tissue becomes more compact, its cavity is obliterated, and it is converted into a body nearly solid. It generally retains, during utero-gestation, a considerable size, and this remark applies especially to the human species, in which it diminishes much more rapidly in size after than before the birth of the child. In some animals it at last wholly disappears; in others, among which is the human species, it always leaves some mark.
In what has now been said regarding the corpus luteum, that body has been described as it is formed in the place of a vesicle which has been burst after fruitful sexual union ; but we may remark that the same series of changes always follows the rupture of an ovarian vesicle from whatever cause that may have proceeded. It is now well known that in some animals the rupture of ovarian vesicles and subsequent changes take place without sexual union merely from the state of heat or venereal excitement of any kind, while in others these phenomena are never observed but as accompaniments of conception. The sow and mare belong to the first of these classes of animals. The rabbit, bitch, ewe, and cow may be mentioned as examples of the second, as also is generally the case in the human female ; but in woman, as in some other females, various circumstances induce us to believe that the rupture of ovarian vesicles and the formation of corpora lutea in their place occasionally happen without sexual union from all those causes which excite greatly the sexual organs; and we are not, therefore, inclined to admit the presence of a corpus luteum, taken alone, as a certain sign of sexual union having oc curred; though conjoined with other signs, the presence of one or more corpora lutea or the appearance of ruptured vesicles must be re garded as good presumptive evidence.
In some of those animals in which vesicles frequently burst without sexual union, there are occasionally very many corpora lutea in the ovary, so as to alter completely its form, and disguise its natural structure, as may frequently be seen in the sow. In those animals again in which sexual union alone brings about the rupture, we at once distinguish the ovary of the unimpregnated animal from that of the one that has had connexion with the male, and we very generally observe an exact correspondence in the number of corpora lutea and the ova or fetuses contained in the uterus ;* and the same correspondence is very frequently found after conception, even in those animals in which corpora lutea are formed without sexual union.
While the corpus luteum, then, is always to be found in the ovary of a pregnant quadruped, the formation of this body is to be regarded as the uniform consequence of the rupture of the ovarian vesicles, whether that rupture shall have been occasioned merely by excitement of the organs, or by productive or unproductive sexual union ; but it is only when conception and pregnancy occur that the corpus luteum attains its full size, and runs through the whole of that series of changes which we have described as peculiar to that body.
We ought not to omit here the mention of a totally different view which has been taken of the corpora lutea, that, viz. of Buffon and Val lisneri,t supported more recently by Sir E. Ilome,t according to which it is held that the corpora lutea exist before the rupture of the vesicles, and are the matrix in which the vesicles and ova are formed.
Two circumstances principally have been brought forward in favour of this hypothesis: 1st, that corpora lutea occur in the virgin state ; and 2d, that they frequently contain vesicles. Now the existence of corpora lutea, we have already stated, in the sow (observed by Sir E.
Home), and even, we are inclined to hold, in the human female, is not necessarily a proof of sexual union having previously occurred, since the rupture of the vesicles may have followed simple excitement of the sexual organs, and might therefore take place either with or without the male ; and in the second place, the occurrence of cavities and vesicular membranes within the corpora lutea is by no means a proof that these cavities are new or Miming ovarian vesicles; on the contrary, there is every reason to regard them as unnatural or the product of disease. But though lately revived upon the above-mentioned grounds, it is long since this hypothesis received the most satisfactory refutation, both from the observa tions of De Graaf and of dialler. Radler in particular traced in the most accurate manner all the steps of the development of the corpus luteum, from the first rupture of the vesicle till its completion: he employed the animals least liable to lead to fallacy in such observa tions; those, viz. in which rupture of the vesicles and formation of corpora lutes is usually produced only by sexual union ; and he always remarked in them an exact corres pondence in the number of foetuses in the gravid uterus with the number of corpora lutea in the ovaries, while at the same time lie found the first appearance of these bodies to take place at a fixed period after sexual union, and their size and structure always to bear an exact relation to the period of utero-gestation at which they were observed.* The uses of the corpora lutes are entirely unknown. The fact that these bodies become larger and remain proportionately of a larger size during pregnancy than when produced in other circumstances (as without sexual union, or after unproductive copulation, or when the product is blighted at an early period of utero gestation,) would seem to indicate some con nexion between the corpora lutca and the development of the foetus in utero. By those who have regarded the corpora lutea as of a glandular nature, they have been supposed to secrete fluids which assist in the nourishment of the firms. We have already stated the reasons for considering such hypotheses as groundless. (See thaws.) Descent if the ovum. Its structure and changes during its pussage.—The attention of accoucheurs in all ages and countries has naturally been directed to the study of the structure of the human ovum and foetus in the more advanced stages of utero-gestation, and a great body of facts has been collected from the examination of aborted products or the gravid uterus of women dying during preg nancy, from which scientific men have acquired an accurate knowledge of the structure of the human firtus and its covering in the ovum during the greater part, and especially in the more advanced period of utero-gestation ; but very little is known of the nature of the egg in the first stages or immediately after concep Lion has occurred. We have, in fact, no direct observations which inform us of what happens to the human ovum immediately after its escape from the ovary, and, indeed, for some little time after its arrival in the uterus, when the parts of the fetus have already begun to be funned in it. This subject has, however, been investigated with considerable success in several maminiferous animals; and although there remain several points which still require eluci dation, yet, from the analogy which is known to exist in the structure of the ovum and foetus of the human species and those of quadrupeds and birds, we are enabled to bring together the detached observations which chance has thrown in our way, and thus to give a connected account of the generative process in man, im perfectly as that process has as yet been observed.