178 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



without indicating any fundamental change in the structure of the cell. 

 Where the cell lias well-developed branches we explain the arrangement by 

 assuming thai tli<' impulse enters the cell-body by one branch and leaves it 

 by another. 



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

 types are found. The first type maybe exemplified by the pyramidal corti- 

 cal cells shown in fig. 70. Here, from a pyramidal body (D) there arise 

 a number of dendrites, while from the lower portion of the cell the axone 



Fig. 71.— Spinal ganglion of an embryo duck; composed of diaxonic nerve-cells (van Gehuchten). 



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

 branches are but two in number and both are medullated. They pass in oppo- 

 site directions, and in this type there are, as a rule, no dendrites. Such 

 are the typical spinal ganglion-cells of the mammal. To understand the 

 arrangement in these cases, recourse must be had to the facts of develop- 

 ment. The second type begins its development as a diaxonic cell, an axone 

 growing from each pole (Fig. 71). In the adult spinal ganglion of the 

 higher mammals, however, such diaxonic cells are rarely found, the great 

 majority having a single axone which soon divides into two branches. 1 



Fig. 72.— Diaxonic changing into monaxonic cells : from the Gasserian ganglion of a developing 



guinea-pig (van Gehuchten). 



Fig. 1'2 beautifully illustrates the phases of this change as seen in a sin- 

 gle section. At first one axone arises from each pole of the ovoid cell-body. 

 Later the cell-body occupies a position at the side of the two axones, which 

 appear to run into one other. Finally the cell-body is separated from the 

 two axones by an intervening stem. The stem has the characters of a medul- 

 lated nerve-fibre, and from the end of it the two original axones pass off as 

 branches. 



1 Dogiel: Anal. Am. Jeva, 1890, Bel. xii. S. 140-152, describes the several kinds of neu- 

 rones which take part in the formation of the spinal ganglion. 



