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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



branches we expect an arrangement of them such that the impulse shall enter 

 the cell-body by one branch and leave it by another. 



On examining the mature nerve-cells of man with this idea in mind, two 

 types are found. The first type may be exemplified by the pyramidal cortical 



FIG. 146. A-D, showing the phylogcnetic development of mature nerve-cells in a series of ver- 

 tebrates ; a-e, the ontogenetic development of growing cells in a typical mammal ; in both cases only 

 pyramidal cells from the cerebrum are shown ; A, frog; B, lizard ; C, rat ; D, man ; a, neuroblast without 

 dendrons ; 6, commencing dendrons ; c, dendrons further developed ; d, first appearance of collateral 

 branches ; e, further development of collaterals and dendrons (from S. Kam6n y Cajal). 



cells shown in Figure 146. Here, from a pyramidal body (D) there arise a 

 number of dendrons, while from the lower portion of the cell the neuron grows 

 out and branches. In the other type the neuron alone grows out. Its branches 



FIG. 147. Spinal ganglion of an embryo duck ; composed of dineuric nerve-cells (van Gehuchten). 



are but two in number and both are medullated. They pass in opposite direc- 

 tions and in this type there are no dendrons. To understand the arrangement 

 in these cases, recourse must be had to the facts of development. The second 



