144 ANATOMY OF THE WOOD RAT 



leg which may or may not be real. It is somewhat at vari- 

 ance with expectations, for according to established beliefs, 

 arboreal or other climbing habits should tend to lengthen the 

 arm, and this subgenus is the least given to climbing of the 

 three. It will be seen that an interpretation of this fact is 

 difficult, for the apparent length of arm in Neotoma may 

 signify merely that its body is somewhat shorter, or there 

 may be conflicting trends which overbalance the influences 

 of a more strictly terrestrial habitat. 



Upper arm 



Humerus, The humerus, which is the bone of the upper 

 arm or brachium, articulates proximad with the scapula 

 at the glenoid cavity, and distad with the radius and ulna. 

 In Homodontomys it averaged 51.7 (50.7 to 52.8) per cent of 

 the functional arm length; in Neotoma, 50.5 (50.4 to 50.8) 

 per cent; and in Teonoma, 52 A (51.7 to 52.7) per cent. There 

 is no variation in this respect with age. The tendency is 

 therefore for the animal which is least scansorial to have the 

 shortest upper arm; but the difference is too sUght to be of 

 great importance. 



The humerus is usually, but not always, slightly thicker 

 and more massive in Teonoma, and perhaps least so in 

 Neotoma. Most of the shaft, including the distal extremity, 

 is straight, but the head, articulating with the scapula, is 

 directed caudad, together with the small portion of the shaft 

 immediately adjoining. The head is usually appreciably 

 the largest in Teonoma. Cranio-laterad of the head 

 is the greater tuberosity, upon which are inserted the 

 supraspinatus, infraspinatus, teres minor, and a part of the 

 pectorahs profundus anterior. From it originate portions of 

 the triceps lateralis and the brachialis also. It must there- 

 fore reflect the influences of six different muscles, and any 

 variation which may occur is correspondingly difficult to 

 interpret. The only one of these muscles noted as showing 



