CENTRAL NERVOUS SYSTEM. 609 



16^, is, when calculated as a cone, approximately 4266 cubic JJL. The neuron 

 from such a cell would have a diameter of at least 2 //, the medullary sheath 

 being included. This gives an area for the cross section of the neuron, of 6.3 

 square p. Thus in the case chosen a portion of the neuron 680 // long would 

 have a volume equal to the cell-body. We may assume this neuron to be 

 15 centimeters 1 50,000 [JL long. Dividing the entire length of the neuron 

 by the length of the piece having the volume of the cell, it appears that the 

 volume of the neuron is 220 times that of the cell-body. 



Repeating the same process with a cell from the lumbar enlargement of the 

 spinal cord, taking a medium cell with a diameter of 46 // and a volume (calcu- 

 lated as a sphere) of 50,000 cubic //, a neuron with a diameter of 10 //, and 

 a length of 100 centimeters, the rektion of the volume of the neuron to that 

 of the cell-body is 1570 to 1. 



This estimate of the volume of the neuron includes, in addition to the axis- 

 cylinder, the enclosing medullary sheath. The volumes of these two portions are 

 approximately equal, so that either the axis-cylinder or the medullary sheath 

 exceeds the cell-body in volume about half as many times as does the entire 

 neuron. It is extremely difficult to estimate the mass of the dendrons. In 

 some instances, as in the cells of the spinal ganglia (Fig. 147) they are absent, 

 while in the large cells of the cerebellum Purkinje's cells they form a mass 

 which must be many times greater than that of the cell-body proper. In most 

 cells, however, the dendrons have at best a mass several times as great as that 

 of the cell-body. 



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

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

 tions 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 mass of the nerve-cells is not proportional to the decrease in the mass 

 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. 



Size of the Neurons in Different Animals. Though the volume of the 

 cell-body and the diameter of the associated neuron are approximately similar 

 in any two animals of different size, as for instance in a bat and in man, it is 

 also evident that the neuron could nevertheless not have the length in the bat 

 that it does in man, and that in this last dimension at least there is a diminu- 

 tion corresponding to the size of the animal. Nevertheless, the volume of the 

 entire cells cell -body plus neuron still remains proportionately very large in 

 the smaller mammals. 



The bearing of this fact on the comparative physiology "of the nervous sys- 

 tem is evident, for, under these conditions, as the volume of the entire nervous 

 system is diminished, the number of cell-elements constituting it must also be 

 1 Die Funktionen der Ganglienzellen des Halsmarkes, Haag, 1891. 



39 



