38 ANATOMY OF THE WOOD RAT 



very similar to that in Homodontomys, although the fibers 

 hardly meet upon the midventral line or extend so far upon 

 the belly; but they occur slightly more caudad in the 

 gluteal region. 



As is usual the pectoral mass is divisible into four por- 

 tions. The names here applied to them are only tentative, 

 for the precise homology of the pectoral muscles in the 

 lower mammals can be established only after extended 

 investigation. 



M. pectoralis superficialis (figs. 7, 29). This portion 

 takes origin along the midventral line from the manubrium 

 to the last sternebra — a distance of 25 mm. The anterior 

 portion is to some extent bipennated, and the muscle is 

 with some difficulty separable along this line. The cranial 

 border skirts a portion of the clavicle, and the whole con- 

 verges to an insertion partly by muscle and partly by ten- 

 don fibers upon the deltoid crest of the humerus and its 

 proximal ridge, partly deep to the deltoid, for a distance of 

 some 12 mm. 



In Neotoma the origin is considerably longer (33 mm.), 

 while in Teonoma the muscle is a trifle thicker as well, with 

 origin 31 mm. in length. In both these subgenera this 

 division entirely hides the pectoralis profundus anterior. 



M. pectoralis profundus posterior (figs. 7, 29). 

 The origin of this slender portion is from the caudal part 

 of the xiphoid process. It at first follows the caudal border 

 of the profundus anterior, and the aponeurotic insertion is 

 upon the deltoid crest and ridge of the humerus deep to 

 that of the superficial division. 



In Neotoma this is separable from the profundus anterior 

 only with difficulty. In Teonoma it is slightly broader, 

 with the cranial border entirely hidden beneath the super- 

 ficial pectoral. 



M. pectoralis profundus anterior (figs. 7, 29) has 

 origin from all the sternebrae (but not the manubrium) 



