The Afferent and Efferent Neurons 



59 



(as visceral pain). The peripheral afferent neurons whose cell-bodies 

 lie in the spinal ganglia, and whose peripheral terminations are in the 

 skin, muscles, and subcutaneous tissues, are receptors, 11 and so are certain 

 other afferent neurons. But not all receptors are neurons: some are epi- 

 thelial cells, as described below [page 61]. 



Receptors have been classified as exteroceptors, enteroceptors, and pro- 

 prioceptors ; the terms being applied to the receptors for external im- 

 pressions, for visceral stimuli, and to receptors ending in muscles, ten- 

 dons and adjacent tissues, respectively. 



THE AFFERENT NEURONS OF THE SPINAL GANGLIA. 



The afferent neuron cells in the spinal ganglia are modifications of 

 bipolar cells, i. e., cells having one axon and one dendrite growing from 



B 



Fig. 44. Bipolar cells in a spinal ganglion of an embryo. Highly magnified. 

 (Schafer, Microscopic Anatomy, after Ramon y Cajal.) A, B, T-cells. E, E, cells 

 still retaining the typical bi-polar form. C, D, F, G, cells in process of transition 

 from the bi-polar to the T-form. 



11 Not all spinal ganglion cells are receptors. Those which send their dendrites to 

 the sympathetic division of the nervous system may connect with receptors lying in 

 this division. 



