" The glands for the secretion of milk are two, one on each side of the middle line of the belly at its lower part. The posterior ends, from which go out the nipples, are on each side of the opening of the vagina in small sulci. They are flat bodies lying between the external layer of fat and abdominal muscles, and are of considerable length, but only one-fourth of that in breadth. They are thin, that they may not vary the external shape of the animal, and have a principal duct, running in the middle through the whole length of the gland, and collecting the smaller lateral ducts, which are made up of those still smaller. Some of these lateral branches enter the common trunk in the direc tion of the milk's passage, others in the con trary direction, especially those nearest to the termination of the trunk in the nipple. The trunk is large, and appears to serve as a reser voir for the milk,* and terminates externally in a projection, which is the nipple. The lateral portions of the sulcus which incloses the nipple are composed of parts looser in texture than the common adipose membrane, which is pro bably to admit of the elongation or projection of the nipple. On the outside of this there is another small fissure, which I imagine is like wise intended to give greater facility to the movements of all these parts. The milk is probably very rich ; for in that caught near Berkeley with its young one, the milk, which was tasted by Mr. Jenner, and Mr. Ludlow, surgeon, at Sodbury, was rich like cow's milk to which cream had been added.
6‘ The mode in which these animals must suck would appear to be very inconvenient for respiration, as either the mother or young one will be prevented from breathing at the time, their nostrils being in opposite directions, there fore the nose of one must be under water, and the time of sucking can only be between each respiration. The act of sucking must likewise
be different from that of land animals ; as in them it is performed by the lungs drawing the air from the mouth backwards into themselves, which the fluid follows, by being forced into the mouth from the pressure of the external air on its surface ; but in this tribe, the lungs having no connexion with the mouth, sucking must be performed by some action of the mouth itself, and by its having the power of expansion." Much stress has recently been laid on the supposed existence which the muscles sur rounding the mammary gland afford in the act of suckling by compressing the gland and ejaculating the milk accumulated in the dilated receptacle above described ; but when we con sider how great the pressure of the surrounding water must be upon the extended surface of the mammary gland, we may readily conceive that when the nipple is grasped by the mouth of the young, and the pressure removed from it by the retraction of the tongue, the milk will be expelled in a copious stream by means of the surrounding pressure alone, independently of muscular aid.
The intimate structure of the mammary gland in the Zoophagous Cetacea is essentially the same as in the Ornithorhynchus, being compo sed of an innumerable quantity of small elon gated ccecal tubes; these are, however, shorter than in the Ornithorhynchus, and their glandu lar parietes are firmer ; they are well shown in the figure of the mammary gland of a young Piked Whale, (Balfflnoptera Rostrata,) given by Muller in his pl. xvii. fig. 2, and according to that author present, after the Ornithorhyn elms, the simplest structure of the mammary gland in the entire mammiferous series of ani mals.]