1916] Holliger: Anatomical Adaptations in the Pocket Gopher 487 



examine the insertion of the triceps, the muscle which makes up 

 over nine-tenths of the volume of the group. The triceps is 

 inserted on the olecranon, and this process in the gopher is much 

 longer than in the other forms. Table 2 shows the length of the 

 olecranon compared to the whole length of the ulna in the seven 

 forms. In the gopher the olecranon composes 25 per cent of the 

 length of the bone; the ground squirrel ranks second in relative 

 length of this process, and the rat third. The three fossorial 

 forms thus have the largest olecranon process. 



Since, as we have seen, the strength of a muscle is directly pro- 

 portional to its volume, we may for a moment let the figures repre- 

 senting the relative volume of group 5 in the various forms (see 

 table 1) stand for the actual pull which it applies to the olecranon. 

 Thus in the gopher the extensors of the forearm would pull with 

 a force equal to 7.9 units, in the ground squirrel 10.5 units, in the 

 rat 9.2 units, etc. Now if there were no mechanical differences in 

 the structure of the limb, these muscles would be considerably less 

 efficient in the gopher than in any of the other forms. But the ulna 

 acts as a lever of the first class with the articulation as its fulcrum, 

 the olecranon as its power arm, and the portion of the bone distal 

 to the articulation as the work arm. The following physical law 

 governs the action of levers: The product of the power by the 

 length of the power arm is equal to the product of the work by the 

 length of the work arm. We may properly use the relative size of 

 the muscles of group 5 to represent the power, and we may easily 

 measure the power arm and the work arm of this lever; hence we 

 may apply the law of levers and compute the actual work delivered 

 at the end of the forearm. This has been done for each of the 

 seven forms, as shows in table 2, 



Table 2 



Table Showing Units of Work Performed by Muscles of Group 5 in the 

 Different Forms, Computed According to the Law of Levers 



Per cent of 



power arm 



(olecranon) 



to whole 



of ulna 



Pocket gopher 25.8 



Ground squirrel 18.3 



Brown rat 17.2 



Jack rabbit 10.6 



Cottontail 12.3 



Belgian hare 15.4 



Chickaree 15.2 



Power arm X 



Power 



= Worki 



arm X 



Work 



.718 



cm. 



7.9 



2.064 



cm. 



2.74 



.732 





10.5 



3.262 





2.35 



.56 





9.2 



2.63 





1.92 



1.21 





11.7 



10.121 





1.39 



1.07 





10.7 



7.605 





1.50 



1.21 





10.1 



6.64 





1.84 



.548 





11.9 



3.048 





2.13 



