486 University of California Publications in Zoology [Vol. 13 



probably the climbing habit would call for a greater use of these 

 muscles. An examination of table 1 shows this functional group to 

 be enormously larger in the rat than in any of the other forms. 

 This is explained, however, by the very great development of the 

 cutaneus maximus muscle, which in the rats dissected composed 

 from 14.4 per cent to 20 per cent of the whole volume of the leg 

 and shoulder. Since much of this muscle has no effect on the 

 movement of the leg, it is evident that group 2 is not larger than 

 would be expected in the rat. The chickaree, our climbing type, 

 shows the next largest muscles of group 2, and in this form the 

 cutaneus maximus is very small. The jack rabbit and the two 

 digging rodents rank close together in size of this group. Contrary 

 to what might be expected, it is relatively smaller in the gopher 

 than in the ground squirrel. But this is accounted for when we 

 consider the point of insertion of the principal muscles of this 

 group. In the gopher the insertion is much farther distad on the 

 humerus than in the ground squirrel, thereby giving a greater 

 mechanical advantage to these muscles in the former rodent than 

 in the latter. This is an example of smaller volume in muscles 

 being compensated for by advantageous insertion. The Belgian 

 hare and the cottontail have group 2 least developed. 



Group 3, the extensors of the upper arm, are best developed in 

 the three rabbits, especially in the cottontail. No explanation of 

 this is apparent, unless it is associated with the fact that the 

 scapulo-humeral ariculation is more movable in the rabbits, as indi- 

 cated by the larger surface of the head of the humerus. In the 

 other forms this group is very nearly of the same relative size. 



Group 4, the flexors of the upper arm, are also largest in the 

 rabbits. Development of this group also would be associated with 

 a more movable humerus such as we should expect to find in types 

 specialized for running. The fact that this group is best developed 

 in the Belgian hare is not explained by the above reasoning, how- 

 ever. Here, as in group 3, the other rodents examined show little 

 difference in the development of these muscles. 



Group 5, the extensors of the forearm, is one which we should 

 expect to find best developed in the fossorial rodents, since these 

 muscles are undoubtedly much used in the act of digging. But 

 table 1 shows this not to be the case; in fact, group 5 is smaller in 

 the gopher than in any of the other forms. The explanation of 

 this seeming lack of correlation is, however, apparent when we 



