42 ANATOMY OF THE WOOD RAT 



forced by the greater mass of the ahmentary tract and the 

 liver. It presents no pecuharities in this genus. The cen- 

 tral semilunar tendon is extensive and into this are radially 

 inserted the fibers of the 



M. sternocostalis, which take origin by muscular 

 bundles interdigitating with the transversalis from the 

 xiphoid process and the sixth to twelfth ribs. In Neotoma 

 origin from the twelfth rib is doubtful, and in Teonoma 

 origin is not only from the twelfth rib, but the thirteenth 

 as well. No other sternocostal muscle was differentiated. 



The two crura are located upon the middorsal line of the 

 body cavity, arising upon either side. Origin is from the 

 first and second lumbar vertebrae, and passing craniad, 

 each is inserted fanwise upon the dorsal portion of the semi- 

 lunar tendon. In Teonoma origin seems to be from the 

 first four lumbar vertebrae. 



The innervation of all these muscles, save the levator 

 scapulae as already mentioned, is by way of the brachial 

 plexus, all of whose branches were not followed in precise 

 detail. 



B. Muscles of the abdomen 



The five muscles of the abdomen are 



Mm. rectus abdominis transversalis 



obliquus abdominis externus cremaster 



obliquus abdominis internus 



M. rectus abdominis (figs. 7, 9, 10, IIB, 32). By 

 virtue of its attachments, a better name for this muscle 

 would be rectus thoraco-abdominalis, but the more usual 

 term is employed. It has origin from the pubis just laterad 

 of the sjonphysis by two slips which decussate with those 

 of the opposite side (fig. IIB). In Homodontomys it was 

 found that the more ventral slip of the right side extended 

 superficial to that of the left, and similarly with the deeper 

 divisions. The superficial slip of the right muscle was 



