196 



HISTOLOGY 



A widely different type of gland, the nephridium, also has motor nerve- 

 endings which control the activities of its epithelial cells. Nerve fibers 



have been found which 

 all but end in the renal 

 cells of the earthworm. 

 But little effort with 

 methylene blue would 

 suffice to bring out the 

 terminal organ which 

 must be closely applied 

 to or actually entered 

 into the cytoplasm. 



Figure 1 74 shows this 

 (After SMIRNOW state as actually demon- 

 strated in the nerve- 

 endings in the frog's renal epithelium. The nerve fibrils here pass 

 into the cytoplasm as varicose fibrils with an irregular course and in 

 sufficient number to control the activities. 



Technic. Methylene blue is the best method for studying these 

 structures. Nitrate of silver will sometimes give good results, but the 

 infra vitam methylene-blue method will satisfy all needs when once the 

 investigator has mastered it and adapted it to his needs. 



FIG. 174. Portion of renal epithelium of frog, showing 

 stimulatory nerve-endings in the cells, 

 in Zool. Anz.) 



LITERATURE 



RETZITJS, G. "Zur Kentniss der motorischen Nervenendigungen," Biol. Unters., 1892. 

 HUBER, G. C., and'DE WITT, L. "A Contribution to the Motor Nerve-Endings," etc., 



Journ. of Cotnp. Neurology, Vol. VII, 1897, p. 169. 

 SIHLER, CHR. "Neue Untersuchungen uber die Nerven der Muskeln mit besonderer 



Berucksichtigung umstrittener Fragen," Zeit.fur Wiss. Zool., Band LXVIII, 1900. 



NEUROGLIA 



The neuroglia tissue is the supporting tissue of the nervous system that 

 has been derived from the ectodermal cells in the early history of the devel- 

 opment of the individual. It is derived from the same layer of cells that 

 the nerve cells themselves are, and its relations can be made clear by looking 

 at a section of the spinal cord of a fish where the ependymal cells, which 

 are neuroglia cells, show a condition that is halfway between a purely epi- 

 thelial form and the internal neuroglia cells found in the brain, where they 

 have assumed a form that would lead one ignorant of their origin to 

 call them connective-tissue cells. Figure 175 represents four neuroglia 

 cells that are graded with reference to their degree of specialization as to 

 branching and internal position. This series is neither ontogenetic or 



