Because of the connections of the serratus magnus with the levator anguli scapulæ, some authors consider it as united with the latter. But as the latter muscle is visible only in the region of the neck (see p. 157), and as it is separately described in man, we prefer to distinguish them from one another. We shall recall the connections to which we have just made allusion when describing the cervical region. Muscles of the Abdomen The abdominal wall is, as in man, formed by four large muscles: the external oblique, the internal oblique, and the transversalis, which form the lateral walls, and the rectus abdominis, situated on each side of the middle line of the abdomen. This latter, because of the general direction of the trunk in quadrupeds, has its superficial surface directed downwards. The arrangement of these muscles closely corresponds to that which we find in the human species. =The External Oblique Muscle= (Fig. 67, 8, 12; Fig. 68, 5; Fig. 69, 9; Fig. 70, 10).--This muscle arises, by digitations, from a number of ribs, which varies according to the species, the number of the ribs being itself variable for each of them, as we pointed out in connection with the osteology of the thorax. Indeed, the great oblique arises from the eight or nine posterior ribs in the dog and the ox, and from the thirteen or fourteen posterior in the horse. It is attached, besides, to the dorso-lumbar aponeurosis. These attachments are arranged in a line directed obliquely upwards and backwards, and the first digitations--that is to say, the most anterior ones--dovetail with the posterior digitations of origin of the great serratus muscle. The fleshy fibres are directed downwards and backwards, and terminate in an aponeurosis which covers the inferior aspect of the abdomen, and proceeds to form the linea alba by joining with that of the muscle of the opposite side, and also to be inserted into the crural arch. This aponeurosis of the external oblique is covered by an expansion of elastic fibrous tissue, which doubles it externally, and which is known as the _abdominal tunic_. This latter is further developed as the organs of the digestive apparatus are more voluminous, and their weight, consequently, more considerable. For this reason, in the large herbivora, as the ox and the horse, this tunic is extremely thick, whereas in the pig, cat, and dog it is, on the contrary, reduced to a simple membrane. Indeed, in these latter, the abdominal viscera being less developed, the inferior wall of the abdomen does not require so strong a fibrous apparatus for supporting them. The great oblique, when it contracts, compresses the abdominal viscera in all circumstances under which this compression is necessary; it also acts as a flexor of the vertebral column.