(2472). The third portion of the urethra is enclosed in the body of the penis, and surrounded by the erectile tissue, of which that organ essentially consists; but in all quadrupeds this part of the canal is not so decidedly continuous with the muscular portion as it appears to be in Man and the generality of Mammalia. In many Ruminants, and in some of the Hog tribe, the muscular division of the canal opens into the upper part of the third or vascular division, in such a manner that a cul de sac occupies the commencement of the vascular bulb of the urethra, as it is called by anatomists, into which the secretion of Cowper's glands is poured without having been previously mixed with the seminal or prostatic fluids. In some Rodents, as, for example, in the Squirrel and the Marmot, the arrangement is still more curious; for the cul de sac of the bulb of the urethra in these creatures, which receives the secretion of Cowper's glands, is lengthened out into a long tube that runs for some distance beneath the proper urethra, and only joins that canal near the extremity of the penis.

(2473). The body of the penis in the Mammalia, as in all other Verte-brata possessed of such an organ, is composed of vascular erectile tissue; but now, besides the corpora cavernosa, which in Reptiles and Birds formed the entire organ, another portion is superadded, destined to enclose the canal of the urethra in a thick erectile sheath, and, moreover, to form the glans, or most sensitive part of the intromittent apparatus.

(2474). The corpora cavernosa are now securely fixed to the bones of the pelvis by two roots or crura; and even in the Cetacea, where no pelvis is met with, the ossa ischii exist, apparently, only for the purpose of giving firm support to the origin of the parts in question. The size of the corpora cavernosa in Man, and many other animals, is of itself sufficient to give the needful rigidity to the parts during sexual excitement; but in some tribes an additional provision is required to ensuro adequate firmness. Thus in Monkeys, Bats, the Carnivora, the Rodentia, and the Balcenidae among Cetaceans, a bone is imbedded in the substance of the male organ, of which it forms a considerable part. Where this bone exists, the corpora cavernosa are proportionately small, and the fibrous walls of the penis are confounded with its periosteal covering.

(2475). The corpus spongiosum, likewise composed of erectile tissue, is quite distinct from the cavernous bodies, and, as we have said before, is only found in the Mammifera. It commences by a bulbous origin that embraces the urethra, and it accompanies that canal quite to the extremity of the penis, where it dilates into the glans.

(2476). The size and shape of the male organ varies, of course, in every genus of quadrupeds, as does the form and texture of the glans. To describe these would lead us into details of too little importance to be noticed in a survey so general as that we are now taking; nevertheless we cannot entirely omit to notice the strange and unaccountable structure met with in some of the Rodent tribes, whereby the penis is rendered a most formidable-looking apparatus, the object of which it is not easy to conjecture, although, as an instrument of excitement, no one will be disposed to deny its efficiency.

(2477). Thus, in the Guinea-pig tribe (Cavia, Illig.), the penis is strengthened by a flat bone that reaches forward as far as the extremity of the gland beneath which is the termination of the urethra; but behind and below the orifice of this canal is the opening of a pouch, wherein are lodged two long horny spikes. When the member is erect, the pouch alluded to becomes everted, and the spikes (fig. 420, d) are protruded externally to a considerable length. Both the erected pouch (b) and the entire surface of the glans are, moreover, covered densely with sharp spines or hooklets; and as though even all this were not sufficient to produce the needful irritation, still further back there are, in some species, two sharp and strong horny saws (c c) appended to the sides of the organ. From this terrible armature of the male Cavies, it would be only natural to expect some corresponding peculiarity in the female parts; but, however inexplicable it may appear, the female vagina offers no uncommon structure.

(2478). We have, in the last place, to examine the generative system of the female placental Mammalia, and thus to trace the development of this important system to its most complete and highest form.

(2479). In the Marsupialia, as the reader will remember, there were still two distinct uteri, that were obviously the representatives of the oviducts of the Oviparous classes. In the Human female, on the contrary, the uterus is a single central viscus, into which the germs derived from the ovaria are introduced through the two "Fallopian tubes" as the oviducts are now designated; but we shall soon see that the viviparous Mammals offer in the anatomical structure of the generative system of the female so many intermediate gradations of form, that we are almost insensibly conducted even from the divided uteri of the Ornitho-rhynchus up to the most elevated and concentrated condition that the uterine apparatus ultimately attains in our own species.

(2480). In the female Rabbit, for example, we have a placental Mammal that in every part of the organization of its reproductive organs testifies its near affinity to the Marsupial type. The ovaria (fig. 421, k, I), although widely different as regards the size of the contained ovules from those of oviparous animals, still retain faint traces of a botryoidal or racemose appearance.

Penis of the Agouti.

Fig. 420. Penis of the Agouti.

Uterus of the Rabbit.

Fig. 421. Uterus of the Rabbit.

(2481). The oviducts (n, o), or the Fallopian tubes as we must now call them, are reduced in their diameter to very small dimensions, and testify by their tenuity how minute must be the ovule to which they give passage. To these succeed the uteri (e, f), still entirely distinct from each other throughout their whole extent, and even opening into the vagina (g) by separate orifices, into which the probes (i, h) have been introduced. As far as its anatomy is concerned, such a uterine apparatus might belong to a marsupial Mammifer; and even in the rest of the sexual parts, obvious relations may be traced between the rodent we are describing and the ovoviviparous quadrupeds.