1 88 VETERINARY PHYSIOLOGY 



must be normal a very small injury to any part may not only 

 entirely alter the reactions or conduct of the animal but may 

 also modify its mental condition. 



Cerebral action frequently goes on without consciousness 

 being implicated ; but so far as we know, consciousness without 

 accompanying cerebral action is unknown, and there is evidence 

 that it is only when the actions of the various parts of the 

 cerebrum are co-ordinated that consciousness is possible. In 

 cases of Jacksonian epilepsy in man, as a result of a small centre 

 of irritation on the surface of the brain, a violently excessive 

 action of the cerebral neurons starts at the part irritated and 

 passes to involve more and more of the brain. In such fits it is 

 found that at first the patient's consciousness is not lost, but that, 

 when a sufficient area of brain is involved in this excessive and 

 iiico-ordinated action, consciousness disappears. 



The study of the action of drugs which abolish consciousness 

 e.g. chloroform and morphine on the dendrites of brain cells 

 suggests a physical explanation of the loss of consciousness. 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 relation- 

 ship with definite dendrites of another set of neurons by 

 their gemmules so as to establish definite paths, the want of 

 co-ordinate relationship established by the general expansion 

 would explain the disappearance of the definite sensations 

 which constitute consciousness. 



It is manifest that the range of consciousness must necessarily 

 be wider where the stored impressions are most abundant, and 

 where the present stimulus most readily calls into action these 

 previous lines of cerebral activity. The storage of impressions 

 is the basis of MEMORY. The power of associating these stored 

 impressions with the present stimulus is the basis of 

 RECOLLECTION. It is the implication of consciousness in this 

 part of brain action which is the basis of mental activity. 



How far the mental action is a mere accompaniment of the 

 physical changes, and how far it can react upon them, is a 

 question which cannot be discussed here But the study of 

 the insane seems to point to the conclusion that the individual 

 may do certain things and have "certain ideas as concomitants 



