CENTRAL NERVOUS SYSTEM. 715 



connections of the cerebellum are mainly with the same side of the bulb and 

 spinal cord. Cephalad of the cerebellum the connection, however, is a crossed 

 one, each cerebellar hemisphere being associated with the contralateral cerebral 

 hemisphere. Throughout these connections, both cephalad and caudad to the 

 cerebellum itself, it appears that there is always a double pathway, and the 

 cerebellum not only sends impulses to, but receives them from, the regions with 

 which it is associated. 



One effect of removal of one half of the cerebellum is to increase the respon- 

 siveness of the cortex of the contralateral cerebral hemisphere to electrical stim- 

 ulation, thereby making it possible with a weaker stimulus to obtain a reaction 

 which could be obtained from the other hemisphere only by a stronger one* 

 When an irritative lesion is made, instead of a merely destructive one, the rota- 

 tion and falling are away from the side of the lesion instead of toward it. 



The experiments altogether show the cerebellum to be closely associated with 

 the proper contraction of the muscles, and this is so directly connected with the 

 maintenance of equilibrium that it is not surprising to find that stimulation or 

 removal of the cerebellar cortex, besides producing nystagmus, may give rise 

 to deviations of the eyes similar to those found on injury to the semicircular 

 canals or stimulation of their nerves in fishes. 1 



PART III. PHYSIOLOGY OF THE NERVOUS SYSTEM 

 TAKEN AS A WHOLE. 



A. WEIGHT OF THE BRAIN AND SPINAL CORD. 



IN attributing a value to the mass of the nervous system we assume that 

 the elements which compose it possess potential energy. This energy varies 

 for any given element in accordance with a number of conditions, but for the 

 moment it will be sufficient to point out that if the mass of the entire system 

 is significant the masses of its respective subdivisions are also significant, as 

 showing in some measure the relative physiological importance of the several 

 parts. 



Changes Dependent upon Age. That the mass of the system varies with 

 age is a matter of common observation. The changes which occur in the mass, 

 although they are specially evident, are not the only changes which take place ; 

 for with the change in mass go hand in hand changes in the relations which 

 the elements bear to one another, and which result in making the organization 

 of the system different at the different periods of life. Moreover, the special- 

 ization of the nerve-elements, in the mammals at least, has been carried to such 

 a point that they are utterly dependent for their full activity on the nutritive 

 system, and the character and amount of the nutrient plasma is a circum- 

 stance of prime importance. Any variation in this factor serves to com- 

 pletely alter the activities of the system, be it never so well organized, and 

 1 Lee : Journal of Physiology, 1893, vol. xv. ; 1894, vol. xvii. 



