298 



THE BODY AT WORK 



arrangement of the fibrils in a cell-body and its dendrites, the 

 appearances which we discover serve to perplex us. They 

 complicate instead of simplifying our mental picture of the 

 conduction of nervous impulses. The coarsest and most dis- 

 tinct neuro-fibrillae are to be found in annelids, the ganglion- 

 cells of a leech, for example, affording excellent preparations. 

 These cells are pear-shaped, with a single stalk. As is usual 

 in invertebrate animals, they do not exhibit separate dendrites 



Cuticle 



Cpithelial 

 cells 



Neuro sensory 

 cell- 



Afferent fibres 



FIG. 20. A GANGLION OP A LEECH. 



Pear-shaped cells are set round a feltwork of nerve- fibrils (neuropil). A neuro-sensory cell is 

 shown with one fibre directed peripherally, branching on the surface ; and one directed 

 centrally, ramifying in the neuropil. Several very slender fibrils from the neuropil pass 

 up the stalk of each ganglion-cell. They join a network near its surface. This net is 

 connected by radiating fibrils with a coarser net which surrounds the nucleus. From the 

 central net a relatively stout fibril passes to muscle-fibres. 



and axon, but dendrites and axon pass out from the cell in the 

 common stalk. The bodies of the cells are set round a felted 

 mass of nerve-filaments, into which their stalks break up. Just 

 beneath the surface of the stalk of one of these cells two or 

 three very fine neuro-fibrillse are to be seen. A single, much 

 coarser fibril occupies its axis. The fine fibrils join a network 

 at the periphery of the cell-body. The thick fibril is connected 

 with a coarser network which surrounds the nucleus. Radiating 

 threads unite the finer with the coarser net. It has been 



