Trapezius ([Fig. 68], 1, 2; [Fig. 69], 1, 2; [Fig. 70], 1, 2).—This muscle, more or less well developed, according to the species, is divided into two portions, of which the names indicate the respective situations—a cervical and a dorsal.
These two parts, considered in the order in which we find them, take their origin from the superior cervical ligament and from the spinous processes of the first dorsal vertebræ. From these different points the fibres are directed towards the shoulder; the anterior are, consequently, oblique downwards and backwards, and the posterior are directed downwards and forwards. They are inserted into the scapula in the following manner: the fibres of the dorsal portion are attached to the tuberosity of the spine; those of the cervical region are also fixed into the same spine, but into a considerably larger surface.
The cervical portion occupies, in the region of the neck, an area relatively smaller than the corresponding portion of the trapezius in man. This diminished degree of development results from the absence, complete, or nearly so, of the clavicle in the animals which we are now considering. We remember, that the trapezius of man is partly inserted into the clavicle, and the disappearance of this latter cannot fail to bring modifications in the general disposition of the corresponding portion of the muscle. There results a disconnection of this latter, and it becomes united to other muscular fibres to form a muscle with which we shall soon have to deal—the mastoido-humeral (see [p. 150]).
Fig. 68.—Myology of the Dog: Superficial Layer of Muscles.
1, Trapezius, cervical portion; 2, trapezius, dorsal portion; 3, superior outline of the scapula; 4, latissimus dorsi; 5, external oblique muscle; 6, rectus abdominis; 7, pectoralis major of the right side; 8, pectoralis minor (sterno-trochinian); 9, 9, mastoido humeral muscle; 10, tendinous intersection, at the level of which is found a rudimentary clavicle; 11, sterno-mastoid muscle; 12, infrahyoid muscles; 13, omo-tracheal or acromio-tracheal muscle; 14, splenius; 15, levator anguli scapulæ; 16, deltoid muscle, spinal portion; 17, deltoid, acromial portion; 18, superior extremity of the humerus; 19, supraspinatus; 20, infraspinatus; 21, biceps cubiti; 22, brachialis anticus; 23, triceps cubiti, long head; 24, triceps cubiti, external head; 25, olecranon process; 26, radialis (anterior extensor of the metacarpus); 27, iliac crest; 28, gluteus maximus; 29, gluteus medius; 30, biceps cruris; 31, semitendinosus; 32, semi-membranosus; 33, gastrocnemius; 34, tensor of the fascia lata; 35, sartorius; 36, fascia lata drawn up by the triceps; 37, the patella or knee-cap; 38, ischio-coccygeal muscle; 39, superior sacro-coccygeal; 40, lateral sacro-coccygeal; 41, inferior sacro-coccygeal.
As specific differences we should add that the trapezius occupies a more or less extensive portion of the median and superior regions of the neck; terminating at a considerable distance from the head in the dog and horse, it, on the contrary, approaches it in the pig and in ruminants. The cervical portion, when it contracts, draws the scapula upwards and forwards, the dorsal portion draws it upwards and backwards. When the trapezius acts as a whole the scapula is raised.
The Latissimus Dorsi ([Fig. 68], 4; [Fig. 69], 5; [Fig. 70], 5).—This muscle arises by an aponeurosis, the so-called dorso-lumbar aponeurosis, from the spinous processes of the last dorsal vertebræ (the seven last in the dog, fourteen or fifteen last in the horse), from the spinous processes of the lumbar vertebræ, and from the last ribs. Its fleshy fibres are directed downwards and forwards, being more oblique in direction posteriorly, and pass on the inner side of the posterior muscular mass of the arm, to be inserted into the internal lip of the bicipital groove of the humerus, or, a little lower down, on the median portion of the internal surface of the same bone. This latter mode of insertion is met with in the horse and the ox.
The anterior fibres cover the posterior angle of the scapula (as in man, where the corresponding angle, but in this case inferior, is covered by the same muscle), and, a little higher up, are in their turn concealed by a portion of the dorsal fibres of the trapezius. It covers, to a greater or less extent, the great serratus muscle. These relations are similar to those found in the human species.