Mammalia: Skin, Muscles, Skeleton 



587 



Fig. 154. Superficial lateral trunk muscles in a reptile (Sphenodon) and a 

 mammal (Felis). The metamerism of the lateral trunk muscles, which is such a 

 striking feature of the lower vertebrates, is reduced and modified in reptiles and 

 almost disappears in adult mammals. (Courtesy, Neal and Rand: "Ghordate 

 Anatomy," Philadelphia, The Blakiston Company.) 



elasticity of the lungs and of the thoracic wall, but it may be hastened 

 or strengthened by contraction of muscles of the abdominal wall. 



In the abdominal wall there are ordinarily no ribs. In rare cases 

 the first lumbar vertebra bears a pair of ribs. They sometimes appear, 

 as an anomaly, in man, being known as "gorilla ribs" because they 

 commonly occur in the gorilla. Aside from the vertebral column there 

 are usually no skeletal parts of any sort between the diaphragm and 

 the pelvic girdle. In the ventral abdominal wall of monotremes and 

 marsupials, however, there is usually a pair of epipubic or marsupial 

 bones supporting the marsupial pouch (when present). They are 

 attached to the pubes. 



In the absence of ribs, the segmental myoseptums are obliterated 

 throughout the greater part of the abdominal wall. The primary 

 segmentation is retained only by certain short muscles passing from 

 one vertebra to the next. Except for the muscles that lie closely adja- 

 cent to the vertebral column, the muscles of the abdominal wall consist 



