DIVEKSE COMPONENTS 357 



of new tissue. The relation of rate of healing to area of wound 

 (fig. 179) is such as to suggest that the healing is proportional to 

 the linear periphery of the wound. In that edge, new tissue prolif- 

 erates and contraction of old tissue occurs. 



In this tissue, cicatrization is a process much slower than recov- 

 ery from stretching or compression, for instance. But compared 

 to replacement of some other human tissues such as cartilage, the 

 velocity quotient of only 0.003/hour seems large. 



(3) Rat viscera. In like manner the rate of hypertrophy of 

 diverse organs of the rat after excision of the symmetrically paired 

 ones may be measured (Addis and Lew, '40). Four of those 

 structural units, namely kidney, adrenal, ovary, and testis, are 

 replaced in a few days to the extent of 56 to 70 per cent of the weight 

 excised ; others such as prostrate and uterine horn are not appreci- 

 ably restored. 



Formation of more tissue, therefore, however it be measured, is 

 faster after the amount of tissue already present has been dimin- 

 ished. The opposite case, in which excess of tissue is transplanted 

 into place, has yet to be studied. What is often thought of as a 

 variety of anatomical adjustment is thus measured in a manner 

 comparable to any of the other components that have been 

 examined. 



§ 128. Excitability of isolated nerve 



But one example is chosen from the study of isolated tissues. It 

 could be also studied, though with possibly diverse results, upon 

 similar tissues in situ. 



Conduction of an impulse along a nerve renders the tissue tem- 

 porarily unresponsive to further stimulation. Recovery of excit- 

 ability may be studied by finding what intensity of stimulus (elec- 

 trical potential) is needed in order that a second impulse be con- 

 ducted. The deficit of excitability is total (load = -100 per cent) 

 just after the first impulse ; the measurement of the reciprocal of 

 potential required identifies how much deficit exists at diverse 

 times thereafter. In a chosen set of conditions the sciatic nerve 

 trunks of frogs recovered half their excitability to electrical stimu- 

 lation in 21 X 10"® hour (0.75 milliseconds). Volleys of impulses 

 conducted at this time are also accompanied by half of the usual 

 action potential (Graham, '35). It is possible that the average 

 axone suffers the same modifications as the nerve trunk. 



