The obliquus internus ex hibits the usual arrangement, arising tendinous and fleshy from the crest of the ileum and pubic ligament, whence it mounts obliquely for wards to be inserted into the cartilages of all the lower ribs as far forwards as the ensi form cartilage of the sternum.
The rectus abdonzinis is much more extensively developed in the horse than in human beings. Arising from the os pubis it passes forwards enclosed in its usual sheath to be inserted into the ensiform cartilage and into the cartilaginous terminations of the third, fourth, fifth, sixth, seventh, eighth and ninth ribs, and also into the sternum between the cartilages of the third and fourth ribs. There are even fleshy fibres derived from this muscle prolonged as far forwards as the articulation between the first rib and the sternum, which, by the old anatomists, was regarded as a dis tinct muscle, and named " musculus in summo thorace situs." In many of the Carnivora the rectus ab dominis is equally remarkable for its great length, and in some species it is even pro longed forwards to the very anterior ex tremity of the sternum. When the recti are thus largely developed the pyramidales do not exist.
Anterior extremity. Muscles of the shoulder. — It may readily be supposed that in the horse and other herbivorous quadrupeds not possessed of a clavicle, and, moreover, remark able for the extreme simplicity of the sync hire of their scapular apparatus, these muscles undergo important modifications in their dis position and attachments, which it will be interesting to investigate. In the human subject the muscles specially appropriated to the movements of the shoulder are eight in number,— viz, the serratus magnus, the pec toralis nzinor, the levator scapula., the rhomboi deus, the trapezizts, the onw-hyoideus, the sub clavius, and the sterno-cleido mastoideus, all of which concur in producing the various move ments of which the human shoulder is sus ceptible. Of these, it will be observed, the six first belong exclusively to the Scapula, except the trapezius, which is inserted ex tensively into the cla.vicle ; whilst the opera tion of the two last is upon the clavicle only.
In quadrupeds the shoulder is furnished with the same muscles as those which are met with in man, only they present differences in their proportions and attachments, which are dependent upon the structure of the skeleton, or the particular requirements of the animal ; and, moreover, they are provided with an additional muscle, of which no vestiges appear in our ovvn bodies. In the horse, the ar rangement or the muscular apparatus of the shoulder is as follows.
The trapezius, in all quadriveds destitute of clavicles, or in which these bones are but imperfectly developed, presents an arrange ment very diff'erent from what exists in such as have the clavicles completely formed : that part which would in the latter case have been the clavicular portion, becomes confounded with the deltoid and with the cleido-mastoid (here a very distinct muscle from the sterno mastoid). From the combination of these three, there usually results a single muscle, which is implanted immediately into the humerus, and which, from its attachments, might be named the nzasto.hunzeralis. It is this muscle which is named by Stubbs the leva'or humeri proprius, and its posterior part musculus ad levatorem accessorius ; and by the French hippotomists nzuscle conzmun de la tete de l'enclosure et du bras. This clavicular por tion of the trapezius is very distinct from the scapular portion, from which it is in many animals separated by the muscle, to be described further on.
In the horse, therefore, the Trapezius may be said to consist of that part only which is called the ascending portion in the human subject, and which is inserted into the pos terior margin of the spine of the scapula, The is present, but the levator anguli scapulce, the cleido-mastoid, and the clavicular portions of the trapezius and deltoid are all replaced by the muscular expansion above described, and which, taking its origin from the mastoid process of the temporal bone and from the transverse processes of some of the superior cervical vertebrw, passes down wards in front of the head of the humerus and descends along the inner surface of the fore arm, into which it is ultimately inserted.