CENTRAL NERVOUS SYSTEM. 175 



toes, thus spanning nearly the entire length of the human body. These are 

 the extreme cases, but as the axones are distributed to all intermediate points 

 both in the central and peripheral system every intermediate length is to be 

 found. 



Volume Relations. — Calculation shows that the volume of the axone of 

 a large motor cell of the spinal cord when it extends from the lumbar 

 enlargement to the foot, may be 1500 times that of the cell-body. This 

 would include in the term axone both the axis-cylinder and the medullary 

 sheath. If either of these is taken alone, the volume is reduced by one-half. 



It is extremely difficult to estimate the volume of the dendrites. In 

 some instances, as in the cells of the spinal ganglia (Fig. 71), they are 

 rarelv present, while in the large cells of the cerebellum (Purkinje's cells) 

 they form a mass which must be several times greater than that of the cell- 

 body proper. In most neurones, however, the dendrites have, at best, a 

 volume as great as that of the cell-body. 



Size of Nerve-cells in Different Animals. — In considering the size and 

 form of cells in man it becomes of interest to determine how far the obser- 

 vations apply to the lower mammals. The facts are briefly these : It can 

 be said that the smaller mammals usually have the smaller nerve-cells, but 

 the decrease in the volume of the nerve-cells is not proportional to the 

 decrease in the volume of the entire body. For example, Kaiser 1 has shown 

 that the cell-bodies occupying the ventral horn in the cervical enlargement 

 of the spinal cord of the bat, the rabbit, and the monkey are in many cases 

 as large or larger than those found in man. 



Though the volume of the cell-body and the diameter of the associated 

 axone are approximately similar in any two animals of different size, as, for 

 instance, in a bat and in man, it is evident that the axones could not have 

 the length in the bat that they do in man, and that in this last dimension at 

 least there is a diminution corresponding to the size of the animal. 



The bearing of this fact on the comparative physiology of the nervous 

 system is evident, for, under these conditions, as the volume <.f the entire 

 nervous system is diminished, the number of cell-elements constituting it 

 must also be diminished, and thus the structure of this system in the smaller 

 mammals becomes numerically simplified. 



Size and Function. — A nerve-cell contains a mass of living substances 

 capable of being broken down and built up chemically, and there is nothing 

 against the inference that the larger the cell the greater is the quantity oi 

 these living substances, and hence the larger the amount of stored energy 

 represented by it. The larger cells are therefore those capable oi setting 

 free the greater amount of energy. The energy-producing changes are to be 

 associated with the cell-body rather than with any of the branches. More- 

 over, the nerve-cdls with large cell-bodies, sending out. as they do. branches 

 which are more voluminous than those the cell-bodies of which are small, 

 furnish a greater amount of material to form the terminal twigs into which 



1 Die Funktumen tin- Oangliemellen <l>:<i Halsmarkcs, Haag, 1891. 



