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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



l-V 



in man. The nerve-cell body is usually ovoid in shape, although this type is 

 much modified in many cases. As will be seen from Figure 143, the diam- 

 eters of nerve-cells range from 10-100 /// and in some instances, in the spinal 

 cord, cells of even larger diameter are found. 



The Structure of the Nerve-cell Body. NissPhas shown that in nerve- 

 cells hardened in strong alcohol there are two substances one which is not 

 stained by a basic aniline dye, and the second which is. The first forms a frame- 

 work continuous with the fibrilke of the nerve-fibre and enclosing the stainable 

 substance in its meshes in small masses or granules. These granules are physio- 

 logically very sensitive, and the study of them under a variety of conditions has 

 already revealed changes in the nerve-cells where none had previously been found. 

 Peculiarities of Nerve-cells. As compared with the other cells of the 

 body, the best developed nerve-cells are of large size, but the nucleus, pro- 

 portionately to the cell-body, is not large, its value decreasing, as a rule, with 

 the increase in the size of the entire cell. The most striking feature of the 

 nerve-cell, however, is the great length to which its chief branch, the neuron, 

 may attain, for in no other tissue does anything like so great a proportion of the 

 cell-substance occur as a branch. The form of cell represented in Figure 144 is 



one in which the neuron shows a very short 

 stem between the cell-body and its terminal 

 twigs. In such an instance the entire exten- 

 sion of the neuron may be less than a milli- 

 meter. With this are to be contrasted those 

 forms in which the neuron is very long and 

 its mass great. What its greatest length 

 may be is easily determined. Within the 

 central system there are cells whose neurons 

 extend from the cerebral cortex to the lumbar 

 enlargement (60 centimeters), and again in the 

 peripheral system there are cell-bodies in the 

 lumbar enlargement of the spinal cord the 

 neurons of which extend to the skin and 

 muscles of the foot a distance of 100 centi- 

 meters. These are the extreme cases, but as 

 the neurons are distributed to all inter- 

 mediate points both in the central and pe- 

 ripheral system, every intermediate length 

 between these and the cells with very short 

 neurons previously mentioned, is to be 

 found. 



Volume Relations. Calculation shows that the volume of the cell-body 

 of a pyramidal cell in the human cerebral cortex having a basal diameter of 



1 fi 0.001 of a millimeter. 



2 Allgemeiner Zeitschriftfur Psychiatric, 1896, Bd. Hi. S. 1147. (A condensed statement of pre- 

 vious work.) 



FIG. 144. A cell with a short neuron 

 giving off many branches. In such a cell 

 the neuron is less in volume than the cell- 

 body. This is the extreme form of the 

 " central cell " (Ram6n y Cajal). D, den- 

 drons ; N, neuron. 



