3. A double uterus, having the appear ance of two horns, which open separately into the vagina ; this is seen in the hare and rabbit, (uterus duplex.) 4. A double uterus, with extraordinary lateral convolutions, is met with in the opossum and kanguroo, (uterus anfrac tuosus.
These various forms undergo different changes in the pregnant state.
The alteration in the simple uterus is, on the whole, analogous to that which occurs in the human female.
The pregnant uterus bicornis suffers a different change in those animals which bear only one at a time, from that which it undergoes in the multipara. The foetus of the mare is confined in its situation to the proper uterus. In the cow it extends at the same time into one of the horns, which is enlarged for its reception. In those, on contrary, which bring forth many young at once, as also in the double uterus of the hare and rabbit, both cornua are divided by contracted portions into a number of pouches cor responding to that of the young ; and where those horns are straight in the un impregnated state, as in the bitch, they become convoluted.
The uterus of the opossum and kangu roo suffers the least change from its usual appearance in the impregnated state. For these strange animals bring their young into the world so disproportion ately small, that they appear like early abortions.
The passage of the foetus, in the opos sum tribe and the kanguroo, from the cavity of the uterus into the false belly, where it adheres by its mouth to the nip ple, presents one of the most singular and interesting phenomena in the whole cir cle of comparative anatomy. Physiolo gists have not yet ascertained, whether the embryo possesses, at any period, a connection with the uterus similar to that which is observed in the other mam malia : but it appears very probable, that the processes, which follow the passage of the ovum from the ovarium, are entire ly different in these animals, from those which take place in the other mammalia. Neither has the precise period, at which the foetus enters the false belly, been hitherto shewn, The following statement of the sub ject, as far as it is at present known, is derived from Mr. Home's paper. (Phil. Trans. 1795.) The uterus and lateral canals, in their pregnant state, are distended with a very adhesive jelly of a bluish white colour ; which also fills the oval enlargements of the Fallopian tubes.
" In the cavity of the uterus," says Air. Home, " I detected a substance which appeared organized ; it was enveloped in the gelatinous matter, and so small as to make it difficult to form a judgment re specting it ; but when compared with the foetus after it becomes attached to the nipple, it so exactly resembled the back bone with the posterior part of the skull, that it is readily recognized to be the same parts in an earlier stage of their for mation." This substance has been represented in a plate ; but the engraving does not, in our opinion, possess the slightest similitude to the parts mentioned by Mr. Home.
The size of the foetus at the time it leaves the uterus is not yet ascertained. The smallest, which has been hitherto found in the false belly, weighed twenty one grains, and was less than an inch in length. In another instance it was " thir
ty-one grains in weight from a mother of fifty-six pounds. In this instance the nipple was so short a way in the mouth,. that it readily dropped out ; we must therefore conclude that it had been very recently attached to it "The foetus at this period had no navel string, nor any remains of there ever having been one ; it could not be said to be perfectly formed, but those parts which fit it to lay hold of the nipple were more so than the rest of the body. The mouth was a round hole, just enough to receive the point of the nip ple ; the two fore-paws, when compared to the rest of the body, were large and strong, the little claws extremely distinct; while the hind-legs, which are afterwards to be so very large, were both shorter and smallerthan the fore ones." " The mode in which the young kan guroo passes from the uterus into the false belly has been matter of much spe culation; and it has even been supposed that there was an internal communica tion between these cavities ; but after the most diligent search, I think I may ven ture to assert that there is no such pas sage. This idea took its rise from there being no visible opening between the uterus and vagina in the unimpregnated state ; but such an opening being very ap parent, both during pregnancy and after parturition, overturns this hypothesis ; for we cannot suppose that the fe=tus, when it has reached the vagina, can pass out in any other way than through the external part." This passage will be facilitated by the power which the animal possesses, of drawing down the false belly to the vulva, which has naturally a considerable projection.
The female organs of generation of birds consist of an ovarium, and an ovi duct, which opens into the cloaca. Its aperture is placed towards the left of that organ. The tube itself is convolut ed, somewhat like an intestine. Its inner coat is furnished with numerous papillae. Its diameter is the most considerable at the cloaca, from which it gradually dimi nishes. It opens towards the abdomen by an expanded orifice, called the in fundibulum ; which is analogous to the fimbriated orifice of the Fallopian tube.
The ovarium, resembling in its appear ance a bunch of grapes, lies under the liver, and contains in a young laying hen about five hundred yolks, varying in size from a pin's head to their perfect mag nitude : the largest always occupy the external circumference of the part. Each yolk is inclosed in a membrane (calyx) which is joined to the ovarium by means of a short stalk or pedicle (petiolus) A white shining line forms on the calyx when the yolk has attained its complete magnitude. The membrane bursting in this part, the contained yolk escapes, and is taken up by the infundibulum in a man ner which we cannot easily conceive. It then passes along the oviduct, and ac quires in its passage the white and shell. The calyx, on the contrary, remains con nected to the ovarium ; but it contracts and diminishes in size, so that in old hens, which have done laying, the whole internal organs of generation nearly dis appear.