apparently not due to real resemblance. What has happened in the Monotremata is, that the prescapular fossa is so enormously expanded that it occupies the whole of the inner side of the blade-bone, while the subscapular fossa which, so to speak, should occupy that situation, has been thus pushed round to the front, where it is divided from the postscapular fossa by a slight ridge only.
The clavicle is a bone which varies much in mammals. It is sometimes indeed, as in the Ungulata, entirely absent; in other forms it shows varying degrees of retrocession in importance; it is only in climbing, burrowing, digging, and flying mammals that it is really well developed.
Fig. 29.—Shoulder girdle, with upper end of sternum (inner surface) of Shrew (Sorex), after Parker, × 7. a, Acromion; c, coracoid; cl, clavicle; ec, partially ossified "epicoracoid" of Parker, or rudiment of the sternal extremity of the coracoid; ''ma'', metacromial process; mss, ossified "mesoscapular segment"; ost, omosternum; pc, rudiment of precoracoid (Parker); ps, presternum; sr1, first sternal rib; sr2, second sternal rib. (From Flower's Osteology.)
In the higher Mammalia the coracoid[[19]] is present, but does not reach the sternum as in the Monotremata. It is known to human anatomists as the coracoid process of the scapula. It has been found, however, by Professor Howes[[20]] and others, that this process really consists of two separate centres of ossification, forming two separate bonelets, which in the adult become firmly ankylosed to each other and to the scapula. These two separate bones have been met with in the embryo of Lepus, Sciurus, and the young of various other mammals belonging to very diverse orders, such as Edentates and Primates. The separation even occasionally persists in the adult. The question is, What is the relation of these bonelets to the coracoid of the Monotremata and to the corresponding regions of reptiles? Professor Howes terms the lower patch of bone the metacoracoid and the upper the epicoracoid;
the former is alone concerned with the glenoid cavity. It must therefore, one would suppose, correspond to the "coracoid" of the Monotremata, while the upper piece of bone is the epicoracoid process of that mammal. The Mammalia, therefore, higher as well as lower, differ from the reptiles in that the coracoid is formed of two bones, the exceptions being, among some other extinct forms, certain of the Anomodontia, a group which it will be recollected is the nearest of all reptiles to the mammals.
Fig. 30.—Distal extremity of the humerus to show Epicondylar Foramina. A, In Hatteria; B, in a Lizard (Lacerta ocellata); C, in the Domestic Cat; D, in Man. c.e, External condyle; c.i, internal condyle. In A the two foramina are developed (at i, the entepicondylar; at ii, the ectepicondylar). The only canal (†) present in the Lizard (B) is on the external ulnar side, in the cartilaginous distal extremity. In Man (D) an entepicondylar process (pr) is sometimes developed and continued as a fibrous band. (From Wiedersheim's Anatomy of Man.)
The Fore-limb.—The humerus is of varying length among mammals. A feature which it sometimes shares with the humerus of lower forms is the presence of an entepicondylar foramen, a defect of ossification situated above the inner condyle of that bone which transmits a nerve. The same foramen and an additional ectepicondylar foramen are found in the ancient reptilian type Hatteria (Sphenodon); it occurs also in the Anomodont reptiles. It is as a rule only the lower forms among mammals which show this foramen; thus it is present in the Mole and absent in the
Horse. The fact that it is occasionally met with in Man is an additional proof of the, in many respects, ancient structure of the highest type of Primate.