328 BRAIN MFX'HANISMS AND LEARNING 



theory of dynamic polarization too absolute a character' (Cajal, 1897). 

 However, it happened that the synapses established by Cajal belonged to 

 the polarization type; this explains why, a short time after, he stated the 

 neuronal doctrine and formulated the law of dynamic polarization. Thus 

 the Spanish author admitted the possibility of dendritic contacts and 

 intennfluences, but later hesitated and disregarded tliem. However, he 

 never denied their existence expressly or categorically (Cajal, 1891, 

 Cajal, 1897; Cajal, 1897). Cajal's conviction was that, if dendro-dendritic 

 connections existed at all, they were rare. He came to think that only 

 occasionally did real dendritic interchanges exist. He therefore denied 

 them an outstanding physiological signiticance as he believed them to be 

 exceptional. In the second volume of his Histolooic chi Systciiii' Ncrveiix, 

 when dealing with the sympathetic ganglia, there is a passage which, once 

 more, shows his uncertainty (Cajal, 191 1). 



Our present aim is to emphasize the existence oi neuronal interrelations 

 through dendritic contacts, and their probable functional significance. 

 Our observations, first upon the cerebellar dendritic glomeruli and retina 

 and then upon many other centres, have shown the presence of (i) a 

 certain closeness, not contact, of dendrites of two or more neurones which 

 held contacts with axonal expansions (axo-dendritic synapses) ; (2) direct 

 dendritic contact in closely grouped microglomeruli, formed exclusively 

 by dendrites belonging to different' neurones; (3) axo-dendro-dendritic 

 glomeruli in which there exists a double type ot close contacts; {a) axo- 

 dendritic and (/)) dendro-dendritic. 



The cerebellar glomeruli are characterized by the diversity of their 

 synapses: (i) according to the origin of the mossy fibres; (2) according to 

 their composition, (<;) axo-dendritic, with the exclusive participation of the 

 mossy fibres, (/>) with the exclusive participation oi the slender collaterals 

 of Golgi type II neurones, {c) with the double participation of rose-shaped 

 endings of the mossy fibres and of the collaterals oi the above-mentioned 

 Golgi II neurones, plus glial fibres with a special terminal device, (d) with 

 only the presence of dendrites of the grains; (3) according to quantitative 

 variations besides qualitative ones; there are glomeruli of very different 

 sizes, as well as glomeruli gathering in more or less numerous groups in 

 which all the aforesaid synapses can be discovered, and also close somato- 

 somatic contacts between grains (Fig. 12). 



All cerebellar grains possess complex synaptic spectra: dendro-dendritic 

 and somato-somatic in their connections with other grains ; axo-dendritic 

 in their connections with vestibular, spinal, bulbar, protuberantial and 

 Golgi II neurones; axo-somatic in connection with this last neurone; in 



