34 Elementary Biology. 



system, while the muscular walls of the vessels assist in the 

 circulation, and by their elasticity regulate the flow. There 

 also the mot^r nerves (vasomotor) determine the degree 

 of contraction of the circular muscle-fibres, and so govern 

 the calibre of the vessel in question. 



(e) Assimilation. The final processes of anabolism 

 which result in the formation of new protoplasm and the 

 repair of tissue-waste are as yet much involved in mystery. 

 No doubt the cells of the various tissues have the power of 

 abstracting or selecting from the blood and lymph supplied 

 to them the special substances which they require. Muscle- 

 cells are thus no doubt able to select the proteids that are 

 needed to form new muscle-cells, and, at the same time, the 

 carbohydrates on which the evolution of muscular energy 

 depends; saliva-secreting cells similarly seize on compounds 

 required to form ptyalin, mucin, and the other constituents of 

 saliva, and similarly for other glands. The nervous system 

 also undoubtedly depends on the blood (the purest in the 

 body) brought to it by the capillaries of the carotid arteries 

 for those compounds needed to repair nervous waste. In 

 the absence of detailed information with regard to the 

 anabolic processes we may summarise the probable nature 

 of these processes and those of katabolism by employing a 

 convenient and probably in the main correct diagram as 

 follows : 



FIG. 191. 



fc 



o> 



IS 



ANABOLISM KATABOLISM 



