The radius and the ulna, which together constitute the fore-arm, are both present in a large number of mammals, but the ulna tends to vanish in the purely walking and digitigrade Ungulates, being present, however, in the more ancient forms of these Ungulates. In Man and in many other mammals the radius can be moved from its normal position and crossed over the ulna; this movement of pronation has been permanently fixed in the Elephant, where the bones are crossed but cannot be altered in position by the contractions of any muscles. Other types agree with the Elephant in this fixation of the two bones.
Fig. 31.—Bones of fore-arm and manus of Mole (Talpa europaea). × 2. c, Cuneiform; ce, centrale; l, lunar; m, magnum; p, pisiform; R, radius; rs, radial sesamoid (falciform); s, scaphoid; td, trapezoid; tm, trapezium; U, ulna; u, unciform; I-V, the digits. (From Flower's Osteology.)
The bones of the wrist show great variation among mammals. The greatest number present are to be seen in such a type as the Mole. Here we have a proximal row, consisting of the scaphoid, lunar, cuneiform, and pisiform, which are arranged in their proper order, beginning with that on the radial side of the limb, that side which bears the first digit. A second row articulates proximally with these bonelets and distally with the metacarpals; the bones composing it are, mentioning them in the same order, trapezium, trapezoid, centrale, magnum, unciform.
The centrale does not, however, really belong to the distal carpal row, and is as a rule situated in the middle of the carpus away from articulation with the metacarpals. It is a bone which is not commonly present in the mammalian hand, but is present in various lower forms, such as the Beaver and Hyrax. It also occurs in such high types as the majority of Monkeys; it is to be found in the Human foetal carpus. Many extinct forms possessed a separate centrale. Its importance in the formation of the interlocking condition of the Ungulate foot is referred to later,
on p. [196]. The only mammal which appears to have the proper five bones in the distal row of the carpus corresponding to the five metacarpals is Hyperoodon, where this state of affairs at least occasionally occurs. The final bone of that series, the unciform, seems to represent two bones fused. Very often the carpus is reduced by the fusion of certain of the carpal bones; thus among the Carnivora it is usual for the scaphoid and the lunar to be fused. It is interestingly significant that these bones retain their distinctness in the ancestral Creodonts. In many Ungulates the trapezium vanishes. The reduction of the toes in fact implies a reduction of the separate elements of the carpus.
As to the digits of the mammalian hand, the greatest number is five, the various supplementary bonelets known as prepollex and postminimus being, it is now generally held, merely supplementary ossifications not representing the rudiments of pre-existing fingers. They may, however, bear claws.[[21]] The number of phalanges which follow upon the metacarpals is almost constantly three in the mammals, excepting for the thumb, which has only two. This is highly characteristic of the group as opposed to reptiles and birds, and the increase in the number of these bones in the Whales and to a very faint degree in the Sirenia is a special reduplication, which will be mentioned when those animals are treated of.
The Pelvic Girdle.—The pelvic girdle or hip girdle is the combined set of bones which are attached on the one hand to the sacrum and on the other articulate with the hind-limb. Four distinct elements are to be recognised in each "os innominatum," the name given to the conjoined bones of each half of the entire pelvis. These are:—the ilium, which articulates with the sacrum; the ischium, which is posterior; the pubis, which is anterior; and finally, a small element, the cotyloid, which lies within the acetabular cavity where the femur articulates. The epipubes of the Monotreme and the Marsupial are dealt with elsewhere (see p. [116]) as they are peculiar to those groups.
Professor Huxley pointed out many years since that while the Eutherian Mammalia differ from the reptiles in the fact that the axis of the ilium lies at a less angle with that of the sacrum,
Ornithorhynchus comes nearest to the reptile in the fact that this axis is nearly at right angles to that of the sacrum. It is particularly interesting to find that this peculiarity of Ornithorhynchus is only acquired later in life, and that the pelvis of the foetus conforms in these angles to the adults of other mammalian groups. In any case, the backward rotation of the pelvis is a mammalian characteristic, and it is most nearly approached among reptiles by the extinct Anomodontia, whose affinities to mammals will be dealt with on a later page (p. [90]). Another peculiarity of the mammalian pelvis appears to be the cotyloid bone already referred to. In the Rabbit this bone completely shuts out the pubis from any share in the acetabular cavity; later it ankyloses with that bone. In Ornithorhynchus the cotyloid or os acetabuli is a larger element of the girdle than is the pubis. In other mammals, therefore, it seems to be a rudimentary structure. But it seems to be a bone peculiar to and thus distinctive of the mammals as compared with other vertebrates. The acetabular cavity is perforated in Echidna as in birds; but in certain Rodents the same region is very thin and only closed by membrane, as in Circolabes villosus.