vin THE FUNCTIONS OF THE BODY 



145 



(Dioncea) closes on its victim. It should 

 be noted that a muscle may contract in the 

 absence of oxgyen or when only a little is 

 available ; if this happens often there is 

 an accumulation of lactic acid, and the 

 proteid frame -work of the muscle goes out 

 of gear, becoming coagulated, as also happens 

 in the rigor mortis after death. 



AVe cannot pass from the muscular sys- 

 tem without recalling that it has an im- 

 portant function as a heat-producer. We 

 are conscious of this " thennogenic ' func- 



o 



tion when we take quick exercise, but heat 

 is produced by muscle apart from contrac- 

 tion. The heat-production is perfected in 

 birds and mammals the warm-blooded 

 animals (better called stenothermal or 

 homoiothermal), where the temperature of 

 the body is kept approximately constant, 

 summer and winter, night and day. The 

 regulation of production and loss of heat 

 by means of the nervous system is brought 

 about in an intricate automatic way, and it 

 gives its possessors a great advantage over 

 the cold-blooded (better poikilothermal) 

 animals, such as reptiles, amphibians, and 

 fishes, whose body-temperature always 

 tends to approximate to that of the out- 

 side world. The heat-regulating or thermo- 

 taxis arrangements of birds and mammals 

 are often imperfect in the young, and we 

 know how quickly some nestlings w r ill die 

 if the mother-bird does not return. It is 

 interesting to find that the three egg-laying 

 mammals (Monotremes) are imperfectly 



FIG. 45. -Ax ENORMOUSLY ENLARGED DIAGRAM OF A 

 SMOOTH OR UNSTRIPED MUSCLE-CELL. (After 

 Schneider.) 



It is spindle-shaped, and while there may be longitu- 

 dinal lines there are no cross striations. Beside the 

 nucleus (N), the fine reticular structure of the cytoplasm 

 is shown. 



11 



