FUNCTIONS OF THE HEMISPHERES. 295 



real sensory ganglia, caused by opening the cranium, and by exposure of these 

 parts, loss of blood, and general depression of the remaining vital power. The 

 movements of the animals are probably reflex, but of a higher kind than those 

 performed through the cord and medulla: thus, a decapitated bird cannot, 

 like the bird from which the cerebrum has been removed, stand on one foot, 

 nor is the foot withdrawn beneath the wing, but it exhibits only convulsive 

 resisting movements ; it can neither walk, fly, nor prune its feathers, which 

 are co-ordinated acts, governed by the cerebellum, and by the pons and other 

 central parts at the base of the cerebrum. 



In man and animals deformed by monstrosity, in which the cerebral 

 hemispheres are especially defective, or even absent, reflex phenomena 

 similar to those already mentioned (p. 294), are observed; for ex- 

 ample, they may suck, swallow, and even cry, but they present no 

 manifestations of any perceptive or other mental quality. That they 

 may have sensation in a feeble degree is probable. 



Disease of one hemisphere in man, so extensive as almost to destroy 

 it, has occurred without any disturbance or diminution of the mental 

 faculties; but when both hemispheres are seriously implicated, such 

 phenomena are always manifested. Slow distension of the hemispheres, 

 by an accumulation of cerebro-spinal fluid in the ventricles of the brain, 

 does not much impair the action of this organ. All sudden injuries, 

 even if slight, act severely as shocks ; but slow suppuration, for ex- 

 ample, has less effect. Inflammation and chronic disease of the gray 

 or cortical matter, are usually accompanied with excitement or lower- 

 ing of the mental faculties, whilst changes in the white or medullary 

 substance, more frequently cause torpor, and loss of voluntary control 

 over the muscles. 



Of the cerebral hemispheres, it may therefore be said, that, by 

 physiological experiment, we have proof that sensations may perhaps 

 be consciously felt without them, but that they are certainly concerned 

 in the perception of sensations, and in the origination of the volitional 

 motorial stimulus or will. It would also seem, on general grounds, 

 that their integrity is essential to the manifestation of that chain of 

 mental acts which may be said to intervene between mere perception 

 and the exercise of the will that is to say, ideation, memory, asso- 

 ciation, and the reasoning processes. As to the emotions, the suppo- 

 sition of their association with some action of the cerebral hemispheres, 

 rests only on general probability. 



The hemispheres appear, however, to be the medium by which, 

 through the channels of special and common sensation, the mind is 

 brought into relation with the outer world, and reacts, through the 

 motor paths, consciously and purposively upon it. This conscious 

 action, which is, in a certain sense, reflex, is intelligence in action, 

 and, as a rule, is proportionate to the size of the cerebrum. It grad- 

 ually diminishes in the descending animal scale, in proportion to the 

 relative decrease of the size and complexity of the brain in compar- 

 ison with the size of the body. The proportionate weight of the cere- 

 brum is not here the only fact to be considered; but also, first, the 

 presence and degree of development of the convolutions, which serve 

 to increase the extent of the gray matter, and, secondly,, the thick- 



