208 ANATOMY OF THE WOOD RAT 



tainty, as they are dependent upon both sorts of stimuli, and 

 in addition, mechanical influence of the ribs. The sterno- 

 fascialis, present in Homodontomys only, seems now to be of 

 no definite use. The serratus posterior superior, of aid in 

 drawing the ribs craniad, is weaker in Neotoma and Teonoma, 

 and the sternocostalis, helping to draw the sternum forward, 

 is smallest in the former animal and largest in the latter. 

 Both are of use in deep breathing. The obliquus abdominis 

 externus is a constrictor of the abdomen, and is larger in 

 both Neotoma and Teonoma, possibly because of the in- 

 creased weight of the viscera due to the more bulky fare of 

 these two animals. This muscle acts also in holding rigid 

 the body during extreme effort of many sorts. 



The significance of the fact that the os penis is smaller in 

 Teonoma than in Neotorna, while it is apparently absent in 

 Homodontomys, is undoubtedly purely phylogenetic. In 

 relation to body length the clavicle is slightly shorter in 

 Homodontomys than in the other two subgenera, but this 

 fact need not be dependent upon any of the muscles attached 

 to it, for it is obvious that the length of this bone must 

 conform to the thoracic width of the animal at this point, 

 and the length varies with differences in body length. 



The scapula, in relation to functional length of arm, is 

 slightly shortest in Neotoma and longest in Teonoma, but 

 the difference is too slight to be of much use to us. Of the 

 muscles which primarily move the scapula in various direc- 

 tions the acromiotrapezius, levator scapulae, and serratus 

 magnus are larger in Teonoma than in the other two sub- 

 genera. But the spinotrapezius was apparentl}^ weaker in 

 both Neotoma and Teonoma, and the anterior rhomboid 

 smallest in the former and largest in the latter. The 

 result is undoubtedly that on the whole, scapular movements 

 are strongest in Teonoma, and to all intents equal in the other 

 two animals. The better definition of the coracoid process 

 of Teonoma should indicate stronger action of the biceps 



