In the angle between the inferior surface of the diaphragm and the lumbar muscles, the two psoas muscles and the quadratus lumborum of the left side, we see the smooth-surfaced kidney, which by this external character, as also by the internal one, of the separation of its cortical or secretory from its medullary or excretory parts, characterizes the class Mammalia. The spleen is in relation with it on the right; to the right of the spleen we have the left end of the stomach, which is less vascular and glandular than the pyloric half, here concealed and overlapped by the left lobe of the liver. From the inferior or convex margin of the stomach the curtain-like omentum, a process of peritoneum found, thus developed, only in mammals, hangs down over the left cornu of the uterus, which is distended with embryos, and over portions of the intestines. Immediately below the kidney and the spleen, the left ovary and Fallopian tube and the upper end of the left cornu uteri are situated. A fibrous band, under which a black bristle is placed, and which is the remnant of the ligament by which the Wolffian body in the foetus was kept in relation with the diaphragm, attaches the ovary and tube to the peritoneal covering of that muscle.
Below the upper end of the left cornu uteri is seen the caecum, which is of less size and complexity than that of Rodents with rootless molars and less varied and nutritious food than these omnivorous members of the order, or than that of those, such as the Squirrels, which live on seeds and have, like most Muridae, rooted molars. It tapers off superiorly into the large intestine, which however in many Rodents is not, when compared with the small intestine, as much inferior in length and larger in calibre and thicker in its walls as its name and the homology of anthropotomy might lead us to expect. Below the caecum we see the cut ends of the veins from the hind-limb, and lower still we see a bristle passed underneath the ureter as it passes forwards to enter the base of the conically contracted bladder. The vagina, rectum, and bladder have, each of them, separate and independent outlets; into those from the two latter organs black bristles have been passed. The flat nail on the rudimentary thumb; the presence of tactile vibrissae above the eyes as well as upon the snout; and of hairs of great coarseness along the mesial dorsal region; the absence of hair from a small area, bifid, as usual in Rodents, in which are the orifices of the nostrils, and which is called the 'muffle;' and its presence between the annulate scales on the tail; are points worthy of notice.
For the relations held by the cerebrum and cerebellum to each other and to the tentorium, see Turner, Proceedings Royal Society of Edinburgh, March 3, 1862.
For the various arrangements observable in the system of the vena azygos, see Milne-Edwards, Lecons sur la Physiologie, vol. iii. p. 598, ibique citata.
For the histology of the Hibernating Gland, see Hirzel and Frey, Z. W. Z. xii. 1862. For that of the Harderian, in Mammals, see Wendt's Monograph, Die Hardersche Druse, 1877; and in Birds, see MacLeod, Bulletin Acad. Royale Sci. Belgique, 1879, pp. 797-810.
For a figure and account of the ligamentum diaphragmaticum in the foetal state, see Kolliker's Entwickelungsgeschichte, p. 961, Fig. 587, 1879; and Tr. Z. S. vol. v. p. 286, 1863.
For an account of the perforation of the clitoris by the urethra in the Cape Mole, see Hunterian Catalogue of the Physiological Series contained in the Royal College of Surgeons, vol. iv. p. 2745; for a similar arrangement in Talpa and Stenops and Lemur, see loc. cit. 2810, 2811, 2812.
For the use of the word 'muffle,' see Waterhouse's Nat. Hist. Mammalia, vol. i. p. 50; vol. ii. pp. 7, 8; Sundevall's Linne's Pecora, Germ. Transl. 1848, pp. 41-432. Skeleton of Common Rat (Mus decumanus).
The skeletons of many of the lower Mammalia bear a general resemblance to those of certain quadrupeds lower in the scale of life in such points as the nearness of the level at which their trunk is carried by their limbs to that of the ground on which they move; and in the maintenance by the long axis of their head, of much the same direction as that of the long axis of their entire trunk. But they invariably present the following distinctive characters, which are as peculiar to the Mammalian class as any of the points furnished by the soft parts, such as the blood-cells, the hairy integument, or the mammary glands. In every Mammalian skeleton each half of the lower jaw is made up of a single mandibular bone on each side, which at birth at least, if not, as it is here, throughout life, is distinct from its fellow of the opposite side, and articulates by a convex facet with the squamosal element of the cranial wall; and the vertebrae in the trunk always differ from those of the different lower Vertebrata in one or more or all of the following points: either in the anchylosis of their several elements, or in the size of their neural canal, or in the shape of the articular ends of their centra, or in the means whereby in the recent state these articular ends are brought into relation with each other.
In the vertebra of a young mammal the neural arch may not have anchylosed with its centrum; but in all such cases two discoid epiphyses belonging to the articular ends of the centrum would also remain unanchylosed, as they fuse with it at a later period than the neural arch, and they furnish a mark as distinctive of the Mammalian class as any other connected with the vertebrae. Some mammals have an opisthocoelian ball and socket articulation between the centre of their vertebrae; and the crocodiles resemble the mammals in having interarticular fibrocartilaginous discs to connect their ball and socket centre-joints instead of synovial joints; but in such cases the greater size of the neural canal or the absence of neurocentral sutures, or the absence of sutures between the body and the lateral processes, would enable us, without having recourse to a microscopic examination of the bony tissue, to identify a vertebra as having belonged to a mammal. In all mammals, except the Cetacea, the maximum number of phalanges in any one digit is limited to three; in nearly all, the number of cervical vertebrae is neither more nor less than seven; and the number of the lumbar vertebrae is never less than two.