126 NERVOUS SYSTEM. 



(2) Fibres of Association are those fibres which connect the 

 convolutions of one hemisphere. These fibres pass in bundles just 

 beneath the cortical gray matter of the convolutions, and it is 

 thought that most of the important convolutions of each hemi- 

 sphere intercommunicate in this way. 



(3) The Medullary Fibres are those which connect the cerebrum 

 and medulla, and are regarded as indirect and direct, according as 

 they do or do not pass to the gray ganglia at the base of the brain. 

 In considering the course of the fibres from the medulla through 

 the crura cerebri it was noted that the motor and sensory fibres 

 were to some extent separated. The fibres pass from the crura to 

 the internal capsule, and here the course of the fibres is twofold : 

 the " direct fibres " pass to the cerebral convolutions through the 

 corona radiata, while the "indirect fibres" pass to the corpora 

 striata and optic thalami, and communicate with ganglion-cells 

 there. 



What is the function of the corpora striata and the optic thalami ? 

 These " basal ganglia," with the other collections of gray sub- 

 stance outside the convolutions, seem to have a controlling influ- 

 ence upon the spinal system. The crura throw their fibres largely 

 to these ganglia, the motor pyramidal tracts to the corpora striata, 

 and the sensory fibres from the lateral and posterior tracts to the 

 optic thalami. It is through these ganglia that all voluntary 

 impulses, except those by the direct medullary fibres, must pass. 

 These basal ganglia communicate through the corona radiata with 

 the convolutions of the cortex, and it is probable that we may 

 regard this part as acting as a middleman to elaborate and co- 

 ordinate the voluntary impulses of the cortex and to act in the 

 matters not requiring the intervention of the higher endowments 

 of the mind. This status of these ganglia is quite theoretical, but 

 the function may be considered as a sort having the properties of 

 both the automatism of the gray matter of the medulla and cord 

 and the voluntary function of convolutions. In this consideration, 

 however, we must not undervalue the communication with the 

 cortex which these basic ganglia possess. 



Do lesions in these basic ganglia cause peripheral symptoms? 



No. So far as has been observed, the corpora striata may be 

 involved by considerable lesions without causing persistent motor 

 or sensory disturbances, and the same may be said of the optic 



