ORGAN AND FUNCTION. 71 



a feather into the common shaft, are short muscles which 

 appear like long ones. These considerations are indis- 

 pensable when we wisli to understand the action of the 

 various muscles of the organism ; it is only by this means 

 that we can estimate the real length of their contractile parts. 



Though the harmony between the form and the function of 

 different muscles is revealed everywhere in the anatomy of the 

 human frame, this harmony becomes much more striking if 

 we compare with each other different species of animals. 

 Comparative anatomy shows us, in species closely allied to 

 each other, a singular difference in the form of certain 

 muscles whenever the function of these muscles varies. Thus, 

 in the kangaroo, essentially a leaping animal, we find an 

 enormous development of the muscles of leaping, the ylutei, 

 the triceps extensor cruris, and the gastrocnemial muscles. 



In birds the function of flight is exercised under very dif- 

 ferent conditions in different species ; so, also, the anatomical 

 arrangement of the muscles which move the wing, the pectoral 

 muscles, varies in a very decided manner in different species. 

 To show the perfect harmony which exists between the func- 

 tion and the organ, it would be necessary to enter into long 

 details of the mechanism of flight. The reader will find, 

 farther on, explanations on this head. We will content our- 

 selves with giving in a few words the differences which have 

 been observed in the movements of the wing, and in the form 

 of the muscles which produce them. 



Every one has remarked that birds which have a large 

 surface of wing, as the eagle, the sea-swallow, &c., give strokes 

 of only a slight extent ; that depends on the great resistance 

 which, a wing of so large a surface meets with in the air. 



Birds, on the contrary, which have but very little wings, 

 move them to a great extent, and thus compensate for the 

 slight resistance which they meet with from the air; the 

 guillemot and the pigeon belong to the second group. If it 

 be admitted that the first-mentioned birds must make 

 energetic but restricted movements, and that the second must 

 inove with less energy, but with greater amplitude of stroke, 

 the conclusion arrived at must necessarily be that the first 

 ought to have large and short pectoral muscles, while in the 



