OH. XV.] PHYSIOLOGY OF NERVE. 163 



contraction called tonus. We shall have to consider this ques- 

 tion again in connection with the plain muscular tissue of the 

 arterioles. 



Involuntary muscle when it contracts undergoes thermal and 

 chemical changes similar to those we have dealt with in the case 

 of the voluntary muscles. 



The nerve-endings in involuntary muscle require a much larger 

 dose of curare to affect them than the end-plates in voluntary 

 muscle. 



The phenomena of rigor mortis in involuntary muscle have 

 never been fully studied. What has been found is that the 

 chemical composition of involuntary muscle differs in no note- 

 worthy manner from that of voluntary muscle, and on death the 

 muscle becomes acid ; such products as carbonic acid and sarco- 

 lactic acid are formed. In the heart, stomach and uterus rigidity 

 has been noted, but in the case of the other involuntary muscles 

 it has never been satisfactorily observed. 



CHAPTER XV. 



PHYSIOLOGY OF NERVE. 



MANY points relating to the physiology of nerve have been 

 already studied in connection with muscle. But there still remain 

 further questions upon which we have hardly touched as yet. 



Classification of Nerves. 



The nerve fibres which form the conducting portions of the 

 nervous system may be classified into three main groups, according 

 to the direction in which they normally conduct nerve impulses. 

 These three classes are : 



r. Efferent nerve fibres. 



2. Afferent nerve fibres. 



3. Inter-central nerve fibres. 



i. Efferent or centrifugal nerves arc those which conduct 

 impulses from the crutral nervous system (brain and spinal 

 cord) to other parts of the body. When for instance there is a 

 wish to move the hand, the impulse starts in the brain, and travels a 

 certain distance down the spinal cord ; it leaves the spinal cord by 



M 2 



