THE VENTRAL MUSCLES 517 



Nerve-Supply. — From the anterior divisions of the thoracic nerves from the 

 fifth to the twelfth. 



Action, — The recti act as flexors of the thorax upon the pelvis or, acting from 

 above, they flex the pelvis on the thorax. They also aid in the compression of the 

 abdominal viscera in defecation and parturition and in strong expiratory efforts. 



Variations. — The origin of the rectus sometimes ascends to the fourth or third rib or even 

 higher. The tendinous inscriptions are probably the persistent representatives of the connective- 

 tissue partitions between certain of the myotomes of which the muscle is composed. They are 

 subject to a certain amount of variation in ninuber, five or six occasionally occurring, while, on 

 the other hand, they may be reduced to two. 



2. Pyramidalis (Fig. 523). 



Attachments. — The pyramidalis is a somewhat variable muscle which arises 

 below from the upper surface of the body of the pubis and from the symphysis and is 

 inserted above into the linea alba, somewhere between the umbilicus and the sym- 

 physis. 



Nerve-Supply. — From the anterior divisions of the eleventh and twelfth 

 thoracic nerves. 



Action. — To tense the linea alba. 



Variations. — The extent to which the muscle is developed varies greatly, its insertion some- 

 times extending well up towards the umbilicus, while, on the other hand, it is not infrequently 

 absent. This latter condition has been estimated to occur in over 16 per cent, of cases. 



{b) THE OBLIQUUS MUSCLES. 



1. Obliquus externus. • 4. Transversalis. 



2. Obliquus internus. 5. Quadratus lumborum. 



3. Cremaster. 6. Intertransversales laterales. 



1. Obliquus Externus (Fig. 524). 



Attachments. — The external oblique forms a muscular sheet in the lateral 

 portions of the anterior abdominal wall. It arises by seven or eight fleshy digitations 

 from the corresponding number of lower ribs, the upper digitations alternating with 

 digitations of the serratus inagnus, while the lower three alternate with those of the 

 latissimus dorsi. The fibres from the lowest ribs pass vertically downward to be in- 

 serted into the crest of the ilium ; the remainder are directed mainly downward and 

 forward and , above, directly forward to join a broad aponeurotic sheet which con- 

 tributes to the formation of the ventral abdominal aponeurosis. 



Nerve-Supply. — From the anterior divisions of the eighth to the twelfth 

 thoracic nerves and from the ilio-hypogastric and ilio-inguinal nerves. 



Action. — Since the external oblique is a curved sheet which passes from the 

 lateral portions of the abdominal wall towards the mid-ventral line, contraction of its 

 fibres will tend to compress the abdorninal contents and so assist in micturition, defe- 

 cation, parturition, and expiration, its action in the last-named process being increased 

 by the power which it possesses of drawing the lower ribs downward. Furthermore, 

 according as it acts from below or above, it will flex the thorax and spinal column 

 upon the pelvis or the pelvis upon" the spinal column, at the same time producing a 

 slight rotation of the thorax to the opposite side and the pelvis to the same side. 

 When the two muscles of opposite sides act together, the rotatory action of each will 

 be neutralized. By the most lateral fibres a lateral flexion of the thorax or pelvis 

 will be produced. 



2. Obliquus Internus (Fig. 525). 



Attachments. — The internal oblique muscle lies immediately beneath the ex- 

 ternal one. It arises from the outer half of Poupart's ligament, from the whole 

 length of the middle lip of the crest of the ilium, and from the lumbo-dorsal 

 fascia. From this extended origin its fibres spread out in a fan-shaped manner, the 

 more posterior ones passing upward and slightly forward to be inserted into the 



