ARRANGEMENT OF THE NERVOUS SYSTEM. 



379 



tion is continued by new fibres originating from the same or adjacent 

 cells. According to this view, the longitudinal tracts consist of fibres 



FIG. 101. 



DIAGRAM OF HUMAN BRAIN IN TRANSVKKSK VERTICAL SECTION. 1. Tuber Annulate; 2,2. 

 CruraCerebri. 3, 3. Internal Capsule. 4, 4. Corona Radiata. o, 6. Cerebral Ganglia. 7. Corpus 

 Callosura. 



which are interrupted in their course by different deposits of gray sub- 

 stance ; so that an impression or impulse conveyed from one region to 

 another is not the same throughout, but is modified in character by the 

 nervous centres which successively receive and transmit it. How many 

 such interruptions there maybe in the transmission of nervous impulses 

 is not known; but it must be considered that, for ordinary motor and 

 sensitive acts, there are, counting from without inward, three successive 

 nervous centres through which they pass ; namely, 



1st. The gray substance of the medullary canal ; 



2d. The cerebral ganglia at the base of the brain ; and 



3d. The convolutions of the hemispheres. 



There are also three sets of fibres in the longitudinal connecting 

 tracts : 



1st. The nerves and nerve roots, connecting the peripheral organs 

 with the gray substance of the medullary canal ; 



2d. The columns of the cord and the crura cerebri, connecting the 

 gray substance of the medullary canal with the cerebral ganglia ; and 



3d. The fibres of the corona radiata, connecting the cerebral ganglia 

 with the convolutions of the hemispheres. 



Thus between the cerebral convolutions and the peripheral organs 

 are two intermediate stations of nervous matter ; namely, the cerebral 

 ganglia and the gray substance of the medullary canal ; and when a 

 nervous impulse passes from the cerebral convolutions to the peripheral 



