THE NEURON 573 



the spinal cord. It has been suggested that the gemmutes at the end 

 of the dendrons are amoeboid in nature, so that this contiguity may be 

 rendered more complete at some times than at others for example, 

 that they may be separated during sleep. There is no evidence in 

 favour of this view. 



The above statement embodies what is known as the neuron theory. 

 Of recent years, the theory has come in for a considerable amount of 

 criticism. Some have affirmed that neuro-fibrillse pass from one neuron 

 into another. Doubt has also been expressed as to whether the neurons 

 are genetically single cells, and whether regenerated nerve-fibres are to 

 be regarded as parts of a single cell. It is obvious that, should either 

 of the above contentions be true, the neuron theory, as originally 

 enunciated, is no longer valid. On the, whole, recent evidence tends 

 rather to conform the theory than otherwise. The development of the 

 nerve cell has been watched in living preparations made from the 

 embryo, and the outgrowth of the axon observed (Fig. 312). 



FIG. 313. PORTION OF RENAL EPITHELIUM OF FROG, SHOWING EFFECTOR (MOTOR) 

 NERVE ENDINGS IN CELLS. (Redrawn after Smirnow, from Dahlgren and Kepner.) 



Those nerve-fibres which conduct inwards towards the nervous 

 system are termed ingoing, or afferent ; those which conduct outwards, 

 the outgoing, or efferent. The nature of a nerve may be ascer- 

 tained (1) by observing the results of its section; (2) by studying 

 the effects produced when the cut ends of the nerve are stimulated. 

 If an afferent nerve be divided, there results a loss of sensation in the 

 area supplied by the nerve. Stimulation of its peripheral end yields 

 no result; stimulation of its central end, on the other hand, calls 

 forth some. If it be sensation, the nerve is termed sensory; if 

 some form of movement, excito-motor ; if inhibition, excito-inhibitory ; 

 if a secretion, excito-secretory. 



Section of an efferent nerve paralyzes some function, the movement 

 of muscle, striped or smooth, secretion of a gland, etc. Stimulation 

 of the central end of such a nerve is without result ; and stimulation 

 of the peripheral end produces the action which it normally excites. 

 This action Tnay either be in a direction of increased activity 

 augmentor or of decreased activity inhibitory. If it excites a 

 muscle, it is termed motor; if the bloodvessels, vaso-constrictor ; if it 



