202 Structure of pishes. 



terrupted respiration, since fishes, when swimming rapid- 

 ly, keep the mcmbrance branchiostigcc closed, and whea 

 nearly exhausted, act violently with their gills. 

 The stnicturc The shortness of the muscular fibres, and the multiplied 

 fonns'a^con" ramifications of the blood vessels, are probably peculiar 

 trast to slow- adaptations for the purpose of gaining velocity of action, 

 moving am- ^hich seems to be invariably connected with a very li- 

 mited duration of it. Such examples form an obvious 

 contrast with the muscular structure of slow-moving 

 animals, and with those partial arrangements where un- 

 usual continuance of action is concomitant. 

 These doc- Since my former communications on the subject of 

 trincs further cylindrical arteries*, another instance of their supplyinff 

 illustrated. / . , . • i. vi r i 



slow-moving muscles, which arc capable of long con- 



I tinned action, has been pointed out to mc by Mr. Ma- 



cartney. It is in the muscles, which act upon the feet 

 and toes of many birds, and seems to be an adaptation for 

 the long exertion of those muscles while they sleep, and 

 also when they alternately retract one foot under the 

 feathers to preserve it fr^ the effects of cold. 



The muscles of the human body, which perform the 

 most sudden actions, have their masses of fibres subdivided 

 by transverse tendons, or are arranged in a penniform 

 direction. The semi-tendinosus, and semi-mcmbranosus 

 of the thigh are thus constructed ; the former having its 

 fleshy belly divided by a narrow /amar, and the fibres of 

 the latter being ranged in a half-penniform manner. The 

 recti abdominis are also divided into short masses by 

 transverse tendons, and all these muscles are conjointly 

 employed in the action of leaping. 



Perhaps these observations may indicate the reason for 

 that diversity in the lengths of various muscles, which act 

 together ; thus, organs of velocity are joined with those 

 of power, and mutually co-operate to produce a simulta- 

 neous effect. 



DESCRIPTION OF PLATE. 



Explanation of The drawing was made from a cod which had been 

 the engraving, coagulated by heat, in a case of plaister of Paris, the skin 

 ♦ " V. 



* Phil Trans. 1800, p.98.-^Al60i8o4,p. 17. 



being 



