ANIMAL MECHANICS. 91 



25.3 grs. Three inches of its tendon weighed in air, 31.4 grs., 

 in water, 4.4 grs. 



The flexor digitorum longus (pedis) weighed 0.83 oz. av. ; 

 one inch of its central portion weighed 96.3 grs., and three 

 inches of its tendon weighed 21.7 grs. 



From these data we obtain — 



Specific gravity of kangaroo muscle, . . 1.076 

 Specific gravity of kangaroo tendon, . . 1 . 1 63 



From the preceding data we obtain — 



Kangaroo Muscles. 



Cross Section of 

 Muscle. 



Cross Section of 

 Tendon. 



Ratio. 



i- Gastrocnemius, . . 



1. 313 sq. in. 



0.0356 sq. in. 



36. 9 



2. Fl. dig. longus, . . 



o.354 „ 



0.0246 ,, 



14.4 



Assuming, as in the case of Man and the Mastiff, that the 

 friction is zero in the muscle that has the highest coefficient 

 of cross section as compared with its tendon ; we find in the 

 kangaroo — 



Fl. dig. long. — Strength of tendon, . . 100 

 . Force of Muscle, ... 39 

 Friction, &c, ... . 61 



It appears from the whole of the preceding investigation 

 that the cross section of a muscle does not bear a constant 

 ratio to the cross section of its tendon, unless the friction 

 exerted upon the muscle and tendon be also constant ; and it 

 is also possible that the strength of the tendon may be some- 

 what greater than what would be absolutely necessary to 

 counterbalance the force of the muscle and friction ; this ad- 

 ditional strength forming, in fact, a surplusage difficult to 

 calculate, but one which cannot be supposed very large, if 

 the principle of economy of material in nature be admitted. 



