=Mastoido-humeralis= (Fig. 66, 3; Fig. 68, 9, 9, 10; Fig. 69, 12; Fig. 70, 15).--One of the most important muscles of the region of the neck in man is the sterno-cleido mastoid. We recollect that, in its inferior part, it is divided into two bundles, one of which arises from the manubrium of the sternum, and the other from the inner third of the clavicle, whence the denominations of the _sternal_ portion and _clavicular_ portion. The muscle formed by the union of these two portions is then directed obliquely outwards, backwards, and upwards, to be inserted into the mastoid process of the temporal bone and the two external thirds of the superior curved line of the occipital bone. Now, the animals which we are here considering have but a rudimentary clavicle or are entirely without it. From the absence of this item of the skeleton there necessarily result modifications in the arrangement of the muscles of this region, which we must at the very outset explain, before undertaking the special study of the muscle which is the subject of the present paragraph. Let us suppose, for the more definite arrangement of our ideas, that the clavicle is altogether absent, although we do find it in a rudimentary state in some animals and completely developed in others (marmot, bat), and we will proceed to indicate what this absence determines. The great pectoral muscle in man arises in part from the clavicle; this origin not being possible in animals which have no clavicle, its attachments, as we have already seen, are concentrated on the sternum. The trapezius in man similarly arises in part from the clavicle; for the reasons above indicated its clavicular fasciculi cannot exist in distinct form in the animals which have no clavicle. The sterno-cleido mastoid, whose inferior attachments we mentioned above, cannot have a clavicular portion. It is the same in the case of the deltoid, which, we know, arises in part from the anterior bone of the shoulder. Of the four muscles which have partial clavicular origins in man, two are known to us in connection with animals--the great pectoral and the trapezius. What has become of the other two, the sterno-cleido mastoid and the deltoid?