34 NERVOUS SYSTEM. 



(D), the corpora striata in front, and the thalami optici 

 behind. Finally, in the posterior portion of the encephalic 

 mass, the cerebellum reproduces on a small scale the preced- 

 ing disposition. (Fig. 12, C. Gray convolutions and rhom- 

 boid body of the cerebellum.) 



We know, moreover, that the prolongations start from the 

 nerve globules, which thus unite them to each other ; in this 

 way a group of these prolongations form a communication 

 in the brain between the superficial and the inner layer of 

 the globules, thus constituting the corona radiata (fibrous 

 cone) ; lying deeper the thalamibs opticus, or the corpus 

 striatum, uniting the middle with the lowest layer. 



The same plan holds in the cerebellum. Collections of 

 nerve prolongations stretch from one portion of the surface, 

 or cortical layer, to the rhomboid body (corpus dentatum) 

 of the cerebellum, then from the latter to the other portions 

 of the encephalon and spinal cord (pedunculi cerebelli, sepa- 

 rated into superior, middle, and inferior). In brief, the 

 encephalon is a very complex system of little continents of 

 gray or central nerve substance, intercommunicating with 

 themselves and spinal cord by numerous commissures. 



The spinal cord, likewise, presents similar commissures; 

 but in this case they are generally longitudinal, and surround 

 the gray centre of the spinal cord with an envelope of white 

 substance (antero-lateral and posterior columns), and then 

 make a communication from one globule to another in the 

 spinal cord, and again between these globules and the brain 

 mass. 



Among these different globules of the centres there are no 

 communications except with these commissures : they simply 

 communicate with each other. There are other communica- 

 tions, placed outside of the nerve centres, with the peripheral 

 parts, by means of nerves in the true sense of the word. 

 The spinal canal (spinal or rachidian, and cephalic portions) 

 alone seems to possess the property of establishing outside 

 communications with the different organs of the economy. 

 All the fibres to be met with in the cerebrum or cerebellum 

 are doubtless real commissures, and only in an indirect way, 

 by the mediation of the cord, can the peripheral nerves be 

 made to accord with the encephalic centres, in order either to 

 produce the sensations (centripetal nerves), or to bring into 

 action the property of volition (centrifugal nerves). 



