FUNCTIONS OF THE CEREBRAL HEMISPHERES 443 



THE FUNCTIONS OF THE CORPUS STRIATUM 

 The mass of grey matter known as the corpus striatum, which consists 

 of the nucleus lenticularis and the nucleus cordatus, is the basal part of the 

 outgrowth from which each cerebral hemisphere is formed and in the lowest 

 vertebrata represents almost the whole of the telencephalon. For many 

 years the corpus striatum was classed with the optic thalamus as the ' basal 

 ganglia/ and these two ganglia were regarded as relay stations between the 

 cerebral cortex and the lower parts of the central nervous system. This view 

 was correct so far as concerns the optic thalamus, in which end all the afferent 

 tracts and from which afferent impressions are carried on by fresh relays of 

 fibres to the cortex. In the higher mammals the motor cortex has a direct 

 connection with the motor nuclei of the bulb and spinal cord through 

 the pyramidal tracts, which are not interrupted anywhere on their course. 

 On destroying the corpora striata degenerated fibres are found running to the 

 optic thalamus, to the red nucleus, and from the latter to the posterior 

 longitudinal bundle. On the other hand the corpus striatum receives fibres 

 from the olfactory tracts and from the optic thalamus. These connections 

 would tend to show that the corpus striatum serves in no way as an inter- 

 mediary between the cortex and the lower parts of the central nervous 

 system, but is an independent mass of grey matter, receiving impulses from 

 the same source as the cortex and sending impulses which may join in the 

 stream of impressions which play upon the lower motor mechanisms of the 

 bulb and cord. 



Isolated excitation of the caudate and lenticular nuclei has no visible 

 effect, provided the current is not so strong as to spread to the adjoining 

 pyramidal fibres in the internal capsule. A study of the evolution of the 

 central nervous system in different classes of animals points to a diminishing 

 importance of these bodies in the normal life of the animal. In the carti- 

 laginous fishes it probably serves to a greater or less degree the same functions 

 in the determination of educated reflexes as the cerebral hemispheres in 

 mammals. In birds the corpus striatum attains its greatest relative develop- 

 ment, the increased powers of adaptation in these animals being apparently 

 procured by development of the corpus striatum instead of as in mammals by 

 increased development of the pallium or cerebral hemispheres. In the 

 monkey Kinnear Wilson found no definite results to follow destruction of 

 the grey matter in these bodies. The animals were, however, only allowed 

 to survive the operation of destruction three weeks, and the same observer 

 has pointed out that destruction of the corpora striata in man may give rise 

 to a morbid condition, characterised by tremor in the execution of willed 

 movements and increased tonicity of the muscles. He therefore ascribes 

 to these bodies, or rather to the sensori- motor mechanism which has its chief 

 meeting- place in their nuclei of grey matter, a steadying effect on the motor 

 system, and places this system by the side of the other systems which we 

 have already studied, namely, the vestibular, the cerebellar, and the 

 pyramidal system. 



