MYOLOGY 137 



as we pointed out in connection with the osteology of the 

 thorax. Indeed, the great obhque 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 hne directed obhquely 

 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 ex- 

 ternally, 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, con- 

 sequently, more considerable. ■ For this reason, in the 

 large herbivora, as the ox and the horse, this tunic is ex- 

 tremely 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. 



The Internal Oblique Muscle.- — This muscle, which is 

 covered by the preceding, arises from the anterior superior 

 iliac spine (external angle in ruminants and solipeds) and 

 the neighbouring parts. From this origin its muscular 

 fibres, the general direction of which is opposite to that of 

 the fibres of the external oblique, diverging, proceed to 

 terminate in an aponeurosis, which contributes to the 

 formation of the linea alba, and to be attached superiorly 

 to the internal surface of the last costal cartilages. It 

 has the same action as the great oblique. What it presents 



