The pectoralis major (sterno-humerien), in the horse, arises, first, from the aponeurosis of the external oblique muscle of the ab domen ; secondly, from the two lower thirds of the sternum ; and, thirdly, from the supe rior part of the sternum for about one-third of its length. The first of these portions winds round to be inserted, into the internal aspect of the head of the humerus; the second ends in a fascia, which descends downwards over the fore-arm, while, the third, running in a transverse direction over the inferior portion, is inserted into the humerus along with the " levator humeri proprius "-bet ween the biceps and the brachimus internus.
In all those rnammalia which are destitute of complete clavicles, even in the Cetacea, there is a part of the sternal portion of the pectoralis major, which is inserted perpen dicularly into the humerus, that joins the cor responding portion of the opposite side to form the muscle common to both arms," by the action of which the two fore-legs are made to cross each other.
The latissintus dorsi in the horse, and in other quadrupeds, exhibits the same arrangement as in the human sub ject : it is, however, in the lower animals powerfully assisted in its action by the massy muscle already described formed by the panniculus carnosus, a strong tendon from which is inserted into the hume rus along with that of the latissimus dorsi. Both are intimately connected with the tendon of the teres major, and from this combination of tendons arises one of the heads of the triceps extensor cubiti.
The supra-spinatus, the infra-spinatus, the subscapularis, the teres nzajor, and the teres minor have, in all quadrupeds, the same ar rangement as in the human subject, the only differences being dependent upon the shape and proportions of the scapula..
The deltoid. in all animals having their clavicles imperfect or wanting, is neces sarily modified in its disposition to a very considerable degree. We have already seen when speaking of the trapezius, that its cla vicular portion is in such cases blended with the anterior division of that muscle: that part only which takes its origin from the scapula remains to be noticed. Where the acromion is well developed, the deltoid may be divided into two portions ; one derived from the acro mion, the other proceeding from the spine and subjacent surface of the scapula: these two portions unite, and, decussating each other, form a common tendon, which is implanted into the deltoid ridge. of the humerus. As
the acromion process diminishes in size, the acromial portion of the deltoid becomes en feebled in like degree, until at length, as in the horse, where there is no acromial projection, that part of the deltoid arising from the spine remains alone. Under these circuinstances, this muscle is directed forwards in nearly the same direction as the infra-spinatus, and, both from its position and office has been named by hippotomists the " abductor longus brachii." The exists even in animals that have no coracoid process, in which case it takes its origin from a little tubercle situ ated upon the superior costa of the scapula. When the biceps arises by two heads, as in the human subject, the coraco-brachialis arises with the longer head by a common tendon ; but when, as is the case in many quadrupeds, the biceps has but one origin from the humerus, the coraco-brachialis is in no way connected with that muscle, Muscles of the The bi ceps, in the generality of quadrupeds, has the same origins as, in the human subject ; one head arising from the neck of the scapula, the other from its coracoid process : these two- heads uniting form a cominon tendon, which is inserted into the tubercle of the radius, and, by an aponeurotic expansion into the fascia covering the muscles of the fore arm ; but where the coracoid process of the scapula is deficient, as in the horse, and in the carnivora generally, the term " biceps " is no longer applicable to this muscle, seeing that it has but one origin from the margin of the glenoid cavity : in the rest of its course it is similarly disposed in all the mammalia.
The brachkezts internus in all quadrupeds has the same arrangement as in the human subject. In the horse, the biceps and the brachixus are by most writers named the " long and short flexors of the forearm." Extensors. — The triceps extensor cubiti is in the horse a muscle of prodigious strength, and consists of three portions similar to those named in the human anatomy the long extensor, the short exten sor, and the brachialis externus (the great extensor,, the middle extensor, and the short extensor of Bourgelat, and other writers on the anatomy of the horse). There is, more over, a fourth portion, derived from the com mon tendon of the latissimus dorsi and teres major, by the intervention of which, it takes its origin from the inferior margin of the sca pula.