THE NERVOUS SYSTEM. 647 



are of three kinds: 1st, Descending fibres, that are made up of the axis- 

 cylinders of the cells of Purkinje carrying impulses down from the cere- 

 bellar cortex. 2d, Ascending fibres, which pass into the granular layer, 

 and there end in a number of very short, finely split fibres, presenting a 

 mossy appearance, so that these are known as the mossy fibres. These 

 connect with the granular cells of this layer. 3d, Ascending fibres, which 

 pass up through the granular into the molecular layer and there break 

 up into a fine network, which interlaces with and coils among the proto- 

 plasmic branches of the cells of Purkinje. 



It will be seen that the arrangements for the transmission and diffu* 

 sion of nerve-impulses and for the cooperation of different cells with 

 each other are extremely complicated and delicate, as would be needed for 

 so important an organ. It is not possible to indicate absolutely by any 

 scheme the course of fibres and the course of impulses through the cere- 

 bellum, but, approximately, it is somewhat like that in the accompany- 

 ing figure (fig. 389A). 



Impulses pass up along those ascending fibres called "mossy" to the 

 granular cells. These cells, being stimulated, send the impulses by their 

 axis-cylinders to the molecular layer, and through their T-shaped divis- 

 ions to the dendrites of the cells of Purkinje. Thence an impulse is 

 send out by the axis-cylinder process of this cell. Other ascending im- 

 pulses are brought up by those fibres which pass to the molecular layer 

 and send their terminals winding around among the dendrites of the cells 

 of Purkinje. Probably impulses pass up also through the ascending 

 fibres, and affect the stellate cells, and through them and their basket- 

 like terminals the cells of Purkinje. 



FUNCTIONS OF THE CEREBELLUM. 



(1.) With the exception of its middle lobe, the cerebellum is itself 

 insensible to irritation and may be all cut away without eliciting signs 

 of pain (Longet). Its removal or disorganization by disease is also gen- 

 erally unaccompanied by loss or disorder of sensibility; animals from 

 which it is removed can smell, see, hear, and feel pain, to all appear- 

 ances, as perfectly as before (Flonrens; Magendie). It cannot, there- 

 fore, be regarded as a principal organ of sensation. Yet, if any of its 

 crura be touched, pain is indicated; and, if the restiform tracts of the 

 medulla oblongata bo irritated, the most acute suffering appears to be 

 produced. 



(2.) Co-ordination of Movements. In reference to motion, the experi- 

 ments of Longet and many others agree that no irritation of the cerebel- 

 lum produces movement of any kind. Remarkable results, however, are 

 produced by removing parts of its substance. Flourens (whose experi- 

 ments have been confirmed by those of Bouillaud, Longet, and others) 

 extirpated the cerebellum in birds by successive layers. Feebleness and 



