Fig. 23.—Episternum of an embryo Mole. (After A. Götte.) cl, Clavicle; es′, central portion of the episternum; es″, lateral portion of the same; r.c, costal ribs; st, sternum. (The figure was constructed from two consecutive horizontal sections.) (From Wiedersheim's Structure of Man.)
The Episternum.—The Mammalia are as a rule to be distinguished from lower Vertebrates by the absence of an episternum, or interclavicle as it is also called. In the Monotremata, however, there is a large
-shaped bone which does not overlie the sternum as in reptiles, but is anterior to it. The relations of this bone to the clavicles seem to leave no doubt that it is the equivalent of the Lacertilian interclavicle or episternum. The Monotremata are not, however, the only mammals in which this structure is to be seen. The Mole in the embryonic condition is
provided with pieces of bone which overlie the manubrium sterni and are attached to the clavicles, and are no doubt to be regarded as the same structure. Probably in many mammals the manubrium will be found to be partly made up of corresponding rudiments. In any case, vestiges of an episternum in the shape of two minute ossicles have been discovered in Man, lying in front of the manubrium. They have been termed ossa suprasternalia. In Man and in the Mole the paired nature of the episternum is clearly apparent. It has been suggested that this structure in its entirety belongs to the clavicles, just as the sternum belongs to the ribs; i.e. that it formed out of the approximated and fused ends of the clavicles. Dr. Mivart[[17]] figured a good many years since a pair of ossicles in Mycetes, lying in one case between the ends of the clavicles and the manubrium sterni, and in another example anterior to the ventral ends of the clavicles. Gegenbaur has figured a
pair of similar bones in the Hamster.[[18]] It is possible that these are to be referred to the same category. It has also been suggested that these supposed episternal rudiments are the vestiges of a pair of cervical ribs.
Fig. 24.—Episternal vestiges in Man. cl, Clavicle, sawn through; es, "episternum" (sternoclavicular cartilage); l′, interclavicular ligament; l″, costoclavicular ligament; m.s, manubrium sterni; o.s, ossa suprasternalia; r.c, first rib; st, sternum. (From Wiedersheim's Structure of Man.)
The Pectoral Girdle.—The skeleton by which the fore-limb is connected with the trunk is known as the Pectoral Girdle. The main part of this girdle is formed by the large scapula, or blade-bone as it is often termed. The coracoidal elements will be dealt with later. The scapula is not firmly connected with the backbone; it is attached merely by muscles, thus presenting a great difference from the corresponding pelvic girdle. The reason for this difference is not easy to understand. On the one hand it may be pointed out that in all running animals at any rate there is a greater need for the fixation in a particularly firm way of the hind-limbs; but, again, in the climbing creatures both limbs would, one might suppose, be bettered by a firm fixation. It must be remembered, however, that in the latter case the same result is at least partly brought about by a well-developed clavicle, which fixes the girdle to the sternum and so to the vertebral column by means of the ribs.