70 U.S. NATIONAL MUSEUM BULLETIN 2 73 



the slip from the fifth was symmetrically absent) . Origin from costal 

 segments two to five is lateral to that of the pectoralis minor, but 

 digitations from the sixth reach the midline. The broad muscular 

 band formed at the lower and ventral border of the rib cage narrows 

 progressively on approaching the pubis. It is inserted on the symphysis 

 and crest behind the falx inguinalis. The muscle has no transverse 

 serrations and the pyramidalis is absent. The costal attachment in 

 Alouatta fusca (Sirena, 1871) on ribs 1-6 and the xiphoid process is 

 not essentially different from the situation in my 13-rib males. 



Nerve supply: Twigs from the anterior cutaneous branches of the 

 lower five thoracic, all of the thoraco-abdominal and the subcostal 

 nerves. 



Function: It is similar to that already assigned to the broad 

 abdominal muscles in previous paragraphs. 



Comparative anatomy: The rectus abdominis appears to offer no 

 major difference among these animals. The wide variability observed 

 in relation to the formation of the rectus sheath has been mentioned 

 in connection with the lateral hypaxial group. 



M. diaphragma (fig. 20) : This is an extensive musculotendinous 

 partition transversely placed between the cranial third and caudal 

 two-thirds of the trunk. The diaphragm is deeply concave on its 

 abdominal side forming a high dome over the abdominal viscera, 

 mainly the liver. The highest part corresponds to its center in agree- 

 ment with the large development of the left hepatic lobe. There is a 

 kidney-shaped tendinous center whose convex ventral margin lies 

 close to the thoracic wall, while its posterior concave border is far 

 from the dorsal body wall. All fibers of the muscle end on this central 

 tendon. The diaphragm arises ventrally from the inner surface of the 

 thoracic outlet formed by the cartilages of the seventh, eighth, and 

 ninth ribs. There is a smaU hiatus behind the xiphoid process, but it 

 does not reach the central tendon. Dorsally, and at the midline a 

 pair of crura are formed in front of the upper lumbar vertebrae. A 

 tendinous right crus arises from the intervertebral disks between L 3 

 and L 4, and L 2 and L 3, as a continuous band next to the anterior 

 longitudinal ligament. The left crus has a similar origin but lies 

 sinistral to the ligament and is thinner. Muscular fibers prolong these 

 tendinous columns and are directed toward the concavity of the 

 central tendon. The crura expand cranially encircling the aorta and 

 the esophagus. A muscidotendinous band joins the crura and separates 

 the vascular from the digestive tube. Digitations are contributed to 

 the diaphragm from the costal processes of L 1 and L 2 and from the 

 last four or five ribs. A costodiaphragmatic sinus can be recognized 

 between the slips from the first lumbar and the last rib. I could not 

 identify any arcuate ligament as such because none of these dorsal 



