HISTOLOGY 147 



Nerves which conduct impulses from receptors toward the central 

 nervous organ are called sensory or afferent. Those which conduct from 

 a central organ to some effector are called motor or efferent. 



Nerves conducting impulses from exteroceptors or proprioceptors to 

 the central organ are somatic sensory (afferent); those conducting from 

 the central organ to the striated muscle of the body wall are somatic motor 

 (efferent). Similarly visceral sensory and visceral motor nerves are 

 distinguished. 



All nervous functions are carried on by protoplasm organized, as 

 always, in cells. To say, as is often done, that nervous tissues consist of 

 nerve cells and nerve fibers is inaccurate. So far as known, every fiber 

 which conducts nervous impulses is developed as an outgrowth from a cell 

 and can function and survive only so long as it remains in physical and 

 physiological continuity with the nucleated Bipolar cells. T-ceii. 



region of the cell of which it is an integral I I _,„.^-^V'/ 



part. Any cell engaged in nervous opera- Lr-^^f^^^^^-^^^'"''^ J^''^^ 



tions, together with all conducting fibers ^/^ ^^ ^.—/^ 



which have grown out from it, is called a /^ ^^ \^i 



neuron. ^ mL 



A central nervous organ is a more or w 



less complex system^ of physiologically ^ J- :^X;;SS'^'7^ 



related neurons serving for the proper cell is transformed into a unipolar 



association, coordination and integration of '^^^-Tlrt^x °r» 5 



nervous impulses. A ganglion is a minor spinal nerves. (From Bremer, 



locaHzed nerve center consisting of the cell- Text-book of Histology.) 

 bodies of neurons together with the adjacent regions of their nerve 

 processes. The neurons of such gangha as the dorsal or spinal gangHa of 

 vertebrates apparently serve only for conduction — that is they do not 

 initiate nerve impulses. 



Neurons are of various types depending on the form of the cell-body 

 and the number of nerve processes (Figs. io8 and 109). Unipolar cells, 

 of comparatively rare occurrence, have a single process ; bipolar neurons are 

 usually spindle-shaped and have a process at each end ; multipolar cells have 

 several processes of which one, the neuraxon (axon or neurite), is rela- 

 tively long, while the short dendrites branch out into fine twigs which end 

 within a short distance of the cell-body. The dendrites seem to be proto- 

 plasmic but the neuraxon is devoid of the granules which characterize the 

 protoplasm of the cell-body. The neuraxon may give off lateral branches 

 (collaterals) and its distal extremity breaks up into fine branches forming 

 the terminal arborization. 



The unipolar cell characteristic of the dorsal gangha of adult vertebrates 

 is apparently a "disguised bipolar" cell. At least in some animals it 

 develops from an embryonic bipolar cell which, in effect, becomes bent 



