The other part (sterno-aponeurotic muscle) is situated more posteriorly, and corresponds to the region known in veterinary anatomy as the inter-fore-limb space, which is limited laterally on each side by the superior portion of the forearm, of which the point of junction with the trunk bears the name ars. Arising from the sternum, as we have above indicated, this portion is directed outwards, to be joined with the terminal aponeurosis of the sterno-humeral, and with that which covers the internal surface of the forearm.
All things considered, the sterno-humeral muscle may be regarded as the representative of the upper fibres of the great pectoral of man, of which the attachments, owing to the more or less complete absence of the clavicle in the domestic mammals, the fibres must be concentrated on the sternum; the sterno-aponeurotic portion then representing the inferior fasciculæ of the same muscle.
Fig. 66.—Myology of the Horse: Anterior Aspect of the Trunk.
1, Pectoralis major (sterno-humeral); 2, pectoralis major (sterno-aponeurotic); 3, mastoido-humeralis; 4, point of the shoulder; 5, sterno-mastoid or sterno-maxillary: 6, inferior portion of the platysma myoides of the neck, divided; 7, triceps cubiti; 8, brachialis anticus; 9, radialis (anterior extensor of the metacarpus); 10, scapular region.
The great pectoral muscle of one side is separated from that of the opposite side along the median line, and especially above and in front, by a groove which is more or less deep, according as the muscles are more or less developed. At the bottom of this groove, suggestive of that which exists in the corresponding region in man, is found, as in this latter, the median portion of the sternum.
The preceding description particularly applies to the arrangement which the great pectoral presents in the horse; in other animals it is marked by some distinctive characters. In the pig, it is inserted into the sternum as far only as the level of the third costal cartilage; in the ox and sheep, it extends as far as the sixth; in the dog, it is attached to the two first sternal pieces only—that is to say, as far as the third costal cartilage. Moreover, in the latter, as in the cat, the two portions which we have indicated are less readily distinguished.
The great pectoral, by its contraction, draws the fore-limb towards the middle line—that is to say, adducts it.
The Pectoralis Minor ([Fig. 67], 6; [Fig. 68], 8; [Fig. 69], 11; [Fig. 70], 12, 26).—This muscle, also called the deep pectoral, is, in animals, larger than the superficial pectoral, therefore certain authors prefer to give to this muscle and the preceding one the names of deep and superficial pectoral respectively. This nomenclature is evidently legitimate, and conforms more to reality, since it does not bring in the notion of dimensions which here is found in contradiction to nomenclature; but, in order to establish more clearly the parallelism with the corresponding muscles in man, we think it better, nevertheless, to give them the names by which it has been customary to designate them in connection with the latter.