THE CENTRAL NERVOUS SYSTEM. 2">7 



dendrites of the cells in the anterior horn, especially those which 

 give off neurites to the anterior roots of the spinal nerves (Fig. 

 236,./). 



The foregoing details may be summarized by means of the accom- 

 panying diagram (Fig. 240), in which the course of a nervous stim- 

 ulus is traced from the organ of sense in, e. g., the skin, to the 

 cortex of the cerebrum, where it is translated into a nervous im- 

 pulse, the course of which is traced to the motor plates of the vol- 

 untary muscles. The reflex mechanism which might at the same time 

 be set into operation is not represented in the diagram, but will be 

 sufficiently obvious from an inspection of Fig. 238. It will be 

 noticed in Fig. 240 that both the sensory stimulus and the motor 

 impulse are obliged to pass through at least two neurons before they 

 reach the ends of their journeys. But the nervous currents are by 

 no means entirely confined to the course marked by the arrows. 

 Impulses may be transmitted in an incalculable number of delicate 

 tracts through the collaterals given off from the neurites within the 

 central nervous system, some of which are indicated in the diagram, 

 and all of which end in teleneurites associated with the teledendrites 

 of, perhaps, several neurons. One of these collateral tracts has 

 already been considered, namely the senso-motory reflexes illus- 

 trated in Fig. 238. 



II. THE CEREBELLUM. 



The cerebellum is subdivided into a number of laminae by deep 

 primary and shallow secondary fissures. The gray matter of the 

 organ occupies the surfaces of these lamina?, while their central por- 

 tions are composed of white matter. The gray matter may be 

 divided into two layers : an external or superficial " molecular 

 layer" and an inner "granular layer" (Figs. 241 and 242). 



The molecular layer contains two forms of nerve-cells : first, the 

 large cells of Purkinje ; second, small stellate cells. 



The cells of Purkinje have large, oval, or pear-shaped bodies lying 

 at the deep margin of the molecular layer. Their dendrites form 

 an intricate arborescent system of branches extending peripherally 

 to the surface of the gray matter, and give off innumerable small 

 teledendrites throughout their course. All these branches lie in one 

 place, perpendicular to the long axis of the lamina in which they 

 are situated, and the teledendrites come into relations with certain 

 longitudinal neurites springing from the cells of the granular layer, 



17 



