CHAP. IL] THE BRAIN. 1093 



the complexity of the chain of which the events in the cortical 

 area are links, and the close tie between sensory factors and the 

 characteristic elements of the motor region. 



In the dog, while removal of almost any considerable portion 

 of the cortex affects sensation, removal of parts in the frontal 

 region producing perhaps less effect than removal of parts in 

 other regions, the loss or impairment of sensation appears to be 

 transient, though having a duration broadly proportionate to the 

 extent of cortex removed ; and when a very large portion of the 

 cortex is removed, some imperfection appears to remain to the end. 

 We have already referred to the case of a dog from which the 

 greater part of both cerebral hemispheres had been removed, but 

 which remained capable of carrying out most of the ordinary 

 bodily movements, and that apparently in a voluntary manner ; in 

 this case the "blunting" of cutaneous sensations was perhaps 

 more striking than the imperfection of movement. It will be 

 worth while to consider the condition of this dog a little closely, 

 on account of the light which it throws on the problem which we 

 are now discussing. 



Clinical experience shews that in man the integrity of the 

 cerebral hemispheres, and of the connection of the hemispheres 

 with the rest of the central nervous system, is essential to the 

 full development of sensations ; and that in this respect each 

 hemisphere is related to the crossed side of the body. A very 

 common form of paralysis or " stroke " is that due to a lesion of 

 some part of one hemisphere (the exact position of the lesion need 

 not concern us now), frequently caused by rupture of a blood vessel, 

 in which the patient loses all power of voluntary movement and 

 all sensations on the crossed side of his body (including the face); 

 he is said to be suffering from hemiplegia, "one sided stroke." 

 Not only do voluntary impulses fail to reach the muscles of the 

 affected side, but sensory impulses, such as those which, started 

 for instance in the skin, would under normal conditions lead to 

 sensations of touch, of heat or cold, or of pain, fail to effect 

 consciousness, when they originate on the affected side; the 

 patient cannot on that side feel a rough surface, or a hot body, 

 or the prick of a pin. For the sake of clearness we suppose the 

 loss of movement and sensation to be complete, but it might of 

 course be partial. Such a case shews we repeat that the integrity 

 of the cerebral hemisphere, and of the connections of that hemi- 

 sphere, we may say of the cortex of that hemisphere, with the 

 other parts of the nervous system, is essential to the development 

 of the sensations; but it does not prove that the cortex of the 

 hemisphere is the " seat " of the sensations, it does not prove that 

 the afferent, and sensory impulses started in the skin, undergo no 

 material change until they reach the cortex and are then suddenly 

 converted into sensations; it only proves that in the complex 

 chain of events by which sensory impulses give rise to full con- 



