STRUCTUEE OF THE. fepiNAL COHD. 



73 



the gray, and that fibers from cells in the gray matter join the white 

 columns. But in amphibians and fishes there are other elements not yet 

 mentioned. Numerous dendrites pass from the ganglion-cells out into the 

 white substance and ramify there (see Fig. 29). The same is true in the 

 embryo of birds and mammals, but in the adults of these classes one seldom 

 finds such dendrites in the white matter. 



Finally it is to be mentioned that in many lower vertebrates true 

 ganglion-cells are found even in the midst of the white matter. There is a 



Fig. 36. — Sections of the spinal cord from large individuals of representative 

 vertebrate classes. A, Section from the crocodile: C. Afrioanus. B, Section from 

 the ostrich: Struthio camelus. C, Section from the shark: Mustelus. 



large group of such cells in the periphery of the dorsal column in Cyclos- 

 tomes and in certain fishes, — the so-called "Dorsal cells"; and also near 

 certain motor roots, large ganglion-cells which probably send their neurites 

 into the motor nerves. A noteworthy condition which exists in birds is still 

 to be described. Here in the lumbar segment of the cord a tissue-mass 

 wedges itself in between the dorsal columns, pushing these so far asunder 

 that it was formerly believed to be a true bifurcation: Sinus rhomboidalis. 



