FUNCTIONS OF THE BRAIN. 823 



Lying between these are found the fibres of the anterior division of the 

 internal capsule, which seem to have no connection with these ganglia. 



Electrical stimulation of these ganglia causes general muscular con- 

 traction on the opposite side of the body. Lesions of either of these 

 ganglia, provided the internal capsule be not injured, do not appear to 

 cause any permanent symptoms, but destruction of the internal capsule 

 causes paralysis of motion or sensibility or both on the opposite side of 

 the body. 



External to the lenticular nucleus is the external capsule, whose 

 function is unknown, as is also the case as regards the claustmm, which 

 is located externally to the external capsule. 



(6) The Optic Thalamus. — Scarcely anything definite is known as 

 to the functions of this organ, since we have been compelled to abandon 

 the theory supported by Carpenter as to its purely sensory nature. 

 Injury to the thalamus of one side sometimes produces partial paralysis 

 on the opposite side of the body, and, again, sometimes after such an 

 injury hemiansesthesia of the opposite side of the body, with or without 

 disturbance of motion, has been observed. Frequently the thalamus may 

 be irritated without producing any evidence of sensation or motion. 

 Since the posterior portion is connected with the origin of the optic 

 nerve it is in all probability concerned in the sense of vision. Together 

 with the corpus striatum, the optic thalamus is perhaps mainly concerned 

 in co-ordinate and complex muscular movements, since the cerebrum 

 may be removed and motion still be normal, provided these basal ganglia 

 are left intact; when, however, they are disturbed normal progression 

 and co-ordinated movements are then rendered impossible. 



The principal difficult}' in determining facts in regard to the func- 

 tions of this part of the brain is that they r do not admit of experimental 

 investigation without the most extensive injury to the other parts of the 

 brain. 



1. The Functions of the Cerebral Lobes. — In man and the higher 

 mammals the cerebral lobes represent the greatest part of the brain-sub- 

 stance, and usually will constitute twelve-thirteenths of the entire weight 

 of the brain. The cerebral hemispheres are composed of an internal 

 white substance, representing the continuations of the fibres coming 

 from below which terminate in an external layer of gray matter. The 

 external matter, the cerebral cortex, is folded into convolutions sepa- 

 rated from each other by fissures, some of which being so marked as to 

 permit of the division of the cerebrum into adjacent lobes. From the 

 cells of the cortex, in all probability, proceed all the motor fibres which 

 are concerned in the production of voluntary movement, and to them 

 come all the fibres from the organs of special and general sense which 

 enable the brain to appreciate external impressions. Some of the fibres 



