THE NERVOUS SYSTEM. 115 



from the other portion by a layer of white material. It is seen on section 

 of the hemisphere. Its horizontal section is wider in the centre than at 

 the end. On the outside is the grey lamina (claustrum). 



Between the corpus striatum and optic thalamus is the tcenia semicir- 

 cularis, a semi-transparent band which is continued back into the white 

 substance of the roof of the descending horn of the ventricle. 



(2) The Optic Thalami are oval in shape, and rest upon the crura 

 cerebri. The upper surface of each thalamus is free, and of white sub- 

 stance; it projects into the lateral ventricle. The posterior surface is also 

 white. The inner sides of the two optic thalarni are in partial contact, 

 and are composed of grey material uncovered by white, and are, as a rule, 

 connected by a transverse portion. 



Functions. The two ganglia, the Corpus Striatum and Optic Thal- 

 amus, are placed between the cerebral convolutions and the crus cerebri 

 of the same side. It is probable that although some of the fibres of the 

 crus pass without interruption into the cerebrum, the majority of the 

 fibres pass into these ganglia; first of all the lower fibres (crusta) into the 

 corpus striatum, and the upper (tegmentum) into the optic thalamus, and 

 then out into the cerebrum. From the position of these bodies, it would 

 be reasonable to suppose that they were interposed in function between 

 the operation of the will on the one hand, and on the other with the sen- 

 sori-motor apparatus below them, and it is believed that this is the case, 

 although the evidence is not exact: the theory that the corpus striatum 

 is the motor ganglion, and that, when injured, the communication be- 

 tween the will ancl the muscles of one half of the body is broken (hemi- 

 plegia), being supported by many pathological facts and physiological ex- 

 periments, and generally received by pathologists. It is found that the 

 cerebral functions are as a rule unimpaired. In the same way the evidence 

 that the optic thalamus is the sensory ganglion depends upon similar 

 observations, that when injured or destroyed, sensation of the opposite 

 side of the body is impaired or lost. In both cases, the parts paralyzed 

 are on the opposite side to the lesions, the decussation of both sets of 

 fibres taking place, as we have seen, below the ganglia. It is a fact, 

 however, that many experiments and pathological observations are op- 

 posed to the above theory, which must therefore be received with caution. 



THE CEREBELLUM. 



The Cerebellum (7, 8, 9, 10, Fig. 326), is composed of an elongated 

 central portion called the vermiform processes, and two hemispheres. 

 Each hemisphere is connected with its fellow, not only by means of the 

 vermiform^ processes, but also by a bundle of fibres called the middle crus 

 or peduncle (the latter forming the greater part of the pons Varolii), while 

 the superior crura with the valve of Vieussens connect it with the cere- 



