186 VETERINARY PHYSIOLOGY 



(4) Hsemorrhage into the brain, or a tumour inside the skull 

 may interfere with the blood supply by pressure and also 

 cause loss of consciousness. 



The study of the action of drugs which abolish conscious- 

 ness — e.g. chloroform and morphine — on the dendrites of 

 brain cells suggests a physical explanation of the condition. 

 It is found that these drugs cause a general extension of 

 the gemmules of all the dendrites ; and, if we imagine 

 that the co-ordinated action of any part of the brain is 

 secured by definite dendrites of one set of neurons 

 coming into relationship with definite dendrites of another 

 set of neurons by their gemmules so as to establish 

 definite paths, the want of co-ordinate relationship estab- 

 lished by the general expansion would explain the dis- 

 appearance of the definite sensations which constitute 

 consciousness. 



5. Time of Cerebral Action. 



The cerebral mechanism takes a very appreciable time to 

 act, and the time varies (1) with the complexity of the action 

 and (2) with the condition of the nervous apparatus. 



Of the time between the presentation of a flash of light 

 to the eye or a touch to the skin and a signal made by the 

 person acted upon when it is perceived, part is occupied in 

 the passage of the nerve impulses up and down the nerves 

 and in the latent period of muscular contraction, but a 

 varying period of something over one-tenth of a second 

 remains, representing the time occupied in the cerebral 

 action {Practical Physiology). 



Continued action of the nerve centres may lead first to a 

 shorteninsr of the reaction time as a result of the facilitation 

 of the passage of the impulses over the synapses (practice), 

 but this is soon followed by a prolongation (fatigue). 

 The latter condition is produced by the action of alcohol, 

 chloroform, and other poisons. 



6. Fatigue of the Cerebral Mechanism. 



Fatigue of the cerebral mechanism is manifested (a) by 

 decrease in the power of attention, comparable to the loss 



