THEORY OF ISOLATED UNITS. 



593 



in certain details of structure, and also in the fact that at once on leaving 

 the cell body they begin to branch dendritically,and the resulting branches 

 terminate in arborisations in the neighbouring grey matter. The 

 dendrons of some nerve cells are beset in their whole course with short 

 and minute processes which have been termed " thorns " or " gemmulse." 1 

 Many nerve cells possess no dendrons ; this is the case with the typical 

 cells of the spinal ganglia, and with those of the corresponding ganglia 

 of the cranial nerves, but it is doubtful whether any nerve cell possesses 

 only dendrons, and is destitute of a nerve fibre process. 2 On the 

 other hand, a nerve cell may possess more than one ; thus the bipolar 

 cells of the spinal ganglia of osseous fishes and the cells of the 



Nereis 



Vertebrata 



FIG. 300. Diagram to show the primitive condition of the afferent nerve cell, 

 and the manner in which it has become altered in the process of evolu- 

 tion of the nervous system. G. Retzius. 



ganglion cochleae possess two, issuing from opposite ends of the cell 

 (bineural cells) ; and cells have been described by Eamon y Cajal 

 in the molecular layer of the cortex cerebri which have three or more 

 (polyneural cells). 



Cajal 3 distinguishes nerve cells into those with a long axis cylinder process 

 (type 1 of Golgi), and those with a short axis cylinder process (type 2 of Golgi), 

 but it is doubtful if these types represent any physiological distinction, as 

 supposed by Golgi, or if, indeed, they are morphologically distinct from one 

 another. 



Theory of isolated units. Neurone theory of Waldeyer Each 



nerve cell, inclusive of the nucleated body and all its processes, irrespect- 



1 See on these structures, A. Hill, Brain, London, 1897, vol. xx. p. 131. 



2 The "amacrin" cells described by Ramon y Cajal in the retina, may be an exception 

 to this rule. See Quain's "Anatomy," vol. iii. pt. 3. 



3 Rev. de den. me'd. de Barcelona, 1891. 



VOL. II. 38 



