296 SPECIAL PHYSIOLOGY. 



ness of the cortical gray substance which follows the convolutions. 

 The relatively larger number of the transverse cornmissural fibres, in 

 the brains of the higher animals, and especially of man, is probably 

 also one element of their superiority. The uses of these commissures 

 are not known, further than that they must serve to associate physio- 

 logically, the opposite halves of the brain. Deficiency of the corpus 

 callosum has been found to be accompanied with want of intellectual 

 power. 



From the anatomical fact already mentioned, that the fibres, ascend- 

 ing from the cord and medulla oblongata along the peduncles of the 

 cerebrum, pass no higher than the gray substance of the optic thalami 

 and corpora striata, which bodies are connected with the gray matter of 

 the cerebral hemispheres by a distinct system of ascending or radiating 

 fibres, and from other considerations, it is probable that the only path 

 of communication between the cerebrum and the outer world, is through 

 that great sensori-motor nervous centre, which is composed of the 

 large masses of gray matter found in the base of the brain, cerebral 

 peduncles, pons Varolii, and in the medulla oblongata, and spinal 

 cord. Even the volitional movements, though dictated by impulses 

 originating in the cerebral hemispheres, are probably excited by stimuli 

 proceeding directly from the sensori-motor nervous axis. In accord- 

 ance with this view, the psychical acts, consequent on sensation, have 

 been thus localized: a sensory impression, reaching the conscious sen- 

 sorium or sensory portion of the sensori-motor apparatus, produces 

 therein a sensation; this sensation, exciting in its turn the cerebral 

 hemispheres, is supposed there to give rise to an idea; the idea, if as- 

 sociated with a feeling of pleasure or pain, becomes an emotion; and 

 such ideas and emotions, when subjected to the intelligence or reason, 

 lead to volitional determinations, or acts of will, which may either 

 produce or restrain particular movements, or may govern and direct 

 the processes of thought. (Carpenter.) The exercise of the will on 

 the voluntary muscles, is also believed, as already mentioned, to be in- 

 direct that is, by actions originating in the hemispheres, and operat- 

 ing on the sensori-motor ganglionic apparatus, at and below the base 

 of the cerebrum, which apparatus then excites the motor nerves. Ad- 

 mitting the correctness of these views, it might be said that the fibres 

 which are known to connect the optic thalami with the cerebral gray 

 matter t)r cortical substance, are ascending or radiating fibres; whilst 

 those which connect the corpora striata with the same parts, are de- 

 scending or convergent, though anatomically, all may be considered 

 as either radiating or convergent. The term "nerves of the internal 

 senses," applied to the white fibres of the hemispheres, by Reil, ex- 

 presses the general notion that they are concerned in offices of con- 

 duction related to the exercise of the emotional and intellectual opera- 

 tions. In further proof of this, it has been remarked, that the will 

 determines only the performance of a given act. or the accomplishment 

 of a certain end or purpose, not the chain of individual and combined 

 movements necessary to arrive at such results. The mind is ignorant 

 of the anatomical positions and connections of the muscles which it 

 employs, or even of their existence; and this is equally true, whether 



