176 VETERINARY PHYSIOLOGY 



IV. THE METHODS BY WHICH THE RECEIVING 

 AREAS HAVE BEEN LOCALISED. 



The determination of the exact parts of the cortex 

 cerebri concerned with the reception of the incoming impres- 

 sions from the different peripheral receptors has proved to be 

 by no means easy, but the combination of various methods 

 has overcome these difficulties and has enabled the localisation 

 to be made with exactness. 



1st. Experimental Methods. — (a) Sensations are the 

 usual accompaniment of the activity of the receiving 

 mechanism. But, in the lower animals, it is not possible to 

 have a direct expression of whether or not sensations are 

 experienced, and therefore, in determining whether removal 

 of any part of the brain has taken away the power of receiv- 

 ing impressions, we have to depend upon the absence of the 

 usual mode of response to the given stimulus. But the 

 absence of this may mean, not that the receiving mechanism 

 is destroyed, but either that the reacting mechanism is out 

 of action, or that the channels of conduction have been 

 interfered with (see tig. 88). 



Thus, if light be flashed in the eye of a monkey, it 

 responds by glancing towards the source of illumination ; 

 and if this movement is absent it may be due to (1) loss of 

 the receiving mechanism ; (2) loss of the mechanism causing 

 the movements ; or (3) interruption of the channels be- 

 tween them. 



Again, it is quite possible that, after removing the 

 receiving mechanism in the cerebrum, external stimuli may 

 lead to the usual response by acting through lower reflex 

 arcs (fig. 88). If, for instance, we suppose the receiving 

 part of the cerebrum connected with the reception of 

 tactile impressions to be entirely destroyed, scratching the 

 sole of the foot may still cause the leg to be drawn up, 

 just as if a sensation had been experienced. Here, although 

 the upper arc is out of action, the lower arc still acts. 



(6) In the lower animals, stimulation of a part of 

 the brain, if it be connected with the reception of impres- 



