Fig. 71.—Myology of the Horse: Panniculus Muscle of the Trunk.
In the same animal the muscular fibres of the panniculus of the trunk arise along a line which connects the stifle-joint to the withers, a line which is, consequently, oblique upwards and forwards. Now, as the fleshy layer is thicker than the aponeurosis, the result is that the mode of constitution of this muscle can be recognised under the skin. Indeed, we can see in some animals, occasionally very distinctly, a slight elevation starting from the region of the abdomen in the neighbourhood of the knee, and thence directed obliquely upwards and forwards. This elevation is produced by the fleshy portion of the panniculus.
In the carnivora, the panniculus of the trunk is not attached to the supraspinous ligament; it is blended with the same muscle of the opposite side, passing over the spinous region of the vertebral column.
From this arrangement results a great mobility of the skin which covers the back. Further, it explains why it is possible to lift up this skin along with the panniculus which it covers, and to which it adheres, throughout the whole extent of the dorso-lumbar column. As we pointed out above, there is also a panniculus muscle of the shoulder and one of the neck. We will deal with them when treating of the regions to which those muscles belong.
The Coccygeal Region
As a sequel to the study of the muscles of the region of the trunk very naturally comes the description of those which, belonging to the region of the coccyx, are destined for the movements of the caudal appendix, of which this latter constitutes the skeleton. The muscles may not seem to be of much importance with regard to external form, but, as they form part of the superficial muscular layer, and as the mass of each is seen in the form of the tail in some animals (the lion, for example), they merit our attention for a moment. A few lines will suffice to give an idea of them. They are: the ischio-coccygeal, superior sacro-coccygeal, lateral sacro-coccygeal, and inferior sacro-coccygeal.
The Ischio-coccygeal ([Fig. 18], 38; [Fig. 69], 33; [Fig. 70], 42).—This muscle, triangular in shape, better developed in the carnivora than in the horse, arises from the spine of the ischium, or from the supracotyloid crest, which replaces this latter in the solipeds and the ruminants. Thence its fleshy mass is directed upwards, expanding as it proceeds to be inserted into the transverse processes of the first two coccygeal vertebræ after insinuating itself between two of the following muscles, the lateral and inferior sacro-coccygeal.
In the dog and cat, the muscle is in great part covered by the great gluteal. In the ox, by a peculiar arrangement of the corresponding region of the [muscles of the thigh]—an arrangement which we will examine in connection with the study of the latter—it is more exposed than in the horse, and gives origin to an outline which corresponds to its general form in the region situated immediately below the root of the tail.