The muscle of the shoulder which is proper to quadrupeds may be named the trachelo acromialis (acromio-trachelien, Cuv., acromio basilaire, Vicq d'Azyr). It arises in the horse from the transverse process of the atlas and of the four following cervical vertebrce (in the generality of quadrupeds from the three upper most only); from this origin it descends towards the shoulder-joint, making its ap pearance externally between the two divi sions of the trapezius, which it separates ; it then spreads out over the acromial portion of the scapula, and descends as far as the middle of the humerus, where it is inserted. lts action will, of course, be to draw the shoulder upwards and forwards. This muscle, which exists in all the mammalia, with the exception of the human species, would seem to be in special relation with the quadrupedal mode of progression ; and, as Cuvier observes, affords a striking example of the difficulty of establishing a good nomenclature in com parative anatomy : in some animals, it derives its origin exclusively from the cranium ; and, in others, from the upper or lower cervical vertebrw. Its mode of insertion is equally various ; in the tapir it is implanted into the aponeurosis which covers the deltoid muscle ; while, in the horse, it has its insertion into the middle portion of the humerus by two aponeurotic tendons, which embrace the bra chialis interims muscle.
The serratus major antieus (scapulo-costien), in the horse, arises from the transverse pro cesses of the third, fourth, fifth, and sixth cervical vertebrce, and also from the external surfaces of the six superior ribs : its origins extending as far backwards as the insertion of the tendons of the sacro-lumbalis : from this extensive origin it passes backwards around the chest to be implanted into the base of the scapula, its insertion occupying nearly half of the internal surface of that bone,. This muscle, Cuvier remarks, is much more extensively developed in quadrupeds than in the human subject ; for, in all other mammalia, except in the orang-outang, it arises not only by digi tations from the ribs, but also from the trans verse processes of the vertebrve of the neck, an arrangement which becomes necessary in animals that walk upon four feet, in order to prevent the scapula from being pushed too far backwards towards the spine. This muscle, in fact, forms, with its fellow on the opposite side, a kind of sling, by which the trunk is suspended. The fact that it is equally exten sive in its attachments in the Monkeys, is an additional proof that the usual mode of pro gression in these animals is on four feet : in these animals, indeed, the serratus magnus derives origins from all the cervical vertebrw, instead of from only a part, as in other mam mifera. In the Cetacew that do not walk, and in the kangaroos which have their ante rior limbs very small, the serratus magnus presents a corresponding feebleness of deve lopment.
The pectoralis minor (serratus minor anticus) is, in the horse, represented by a muscle, which, arising from the sternum and from the first, second, third, and fourth ribs near their sternal terminations, runs upwards and back wards to be inserted into the superior costa of the scapula near the base of that bone; it also contracts tendinous attachments with the aponeurotic covering of the teres minor and other scapular muscles.
The rhomboideus arises, in the horse, entirely from the ligamentum nuchw, and from the spines of the anterior dorsal vertebrw, whence it runs outwards to be affixed to the base of the scapula.
In monkeys and in the carnivorous qua drupeds the rhomboideus is continued up wards as far as the occiput, whence it derives an extensive origin ; the occipital portion, indeed, is, in the Carnivora, separated from the rest so as to form a distinct muscle, called by some writers the occipito-scapularis, and, by Cuvier, "rhomboide de la rite." The omo-hyoideus is, generally, wanting in animals whose scapula presents no coracoid process : neither can there be any subclavius in animals that do not possess a clavicle : in the horse, however, the former of these is represented by a strong muscular fasciculus.
In this place it may be proper to notice the muscle named by the human anatomist sterno cleido-mastoideus ; but which, in the lower ani mals, is represented by two distinct muscles. One of these, The sterno-mastoideus, or, as it might be named, sterno-maxillaris, arises, in the horse, from the anterior end of the sterntim, and, running forwards strong and fleshy, is inserted by a flat tendon into the inferior maxilla under neath the parotid gland, sending, however, another tendon to be implanted into the root of the mastoid process.
The cleido-mastoideus, always a distinct muscle from the preceding, is, as we have seen above, in the horse and other non-claviculate quadrupeds, confounded with the clavicular portions of the trapezius and deltoid.
Muscles inserted into the hun2erus. —The movements of the humerus in the human body are effected by two sets of muscles : one derived from the trunk, the other from the framework of the shoulder. The former are the pectoralis major and latissimus dorsi ; the latter, the supra-spinatus, the infra-spinatus, and the subscapularis proceeding from the surfaces of the scapula and the deltoid, teres minor, teres major, and coraco-brach ial is, which take their origins from the processes. In the horse and in other quadrupeds, various cir cumstances render modifications in the ar rangement of these muscles indispensable.