28 

 centres homologous v?ith the spina] centres, likewise divisible 

 into somatic and splanchnic groups. 



The impetus given by Gaskell has led, ultioiately, to the 

 modern conception that a spinal nerve embraces neurones derived 

 from four distinct sources, with as many different distribu- 

 tions. There are, then, to be distinguished in a spinal nerve: 

 (a) Somatic motor fibres. These neurones have their cell- 

 bodies situated in the ventral cornu of the cord, their axones 

 emerge through the ventral root, and they are distributed to 

 the body musculature, (b) Somatic sensory neurones, the cell- 

 bodies of which comprise the dorsal ganglion, and their fibres 

 connect peripheral end-organs with the dorsal cornu of the 

 cord through the medium of the dorsal root. These two divis- 

 ions of a spinal nerve comprise the principal number of fibres 

 in the two roots, (a) Viscero-motor; these fibres take their 

 origin from a group of cells, the paracentral nucleus of Onuf 

 and Collins ('9B), lying lateral to the canalis centralis; 

 they emerge through both the ventral and the dorsal roots, 

 and are distributed to the non-striated muscles of the viscera. 

 (i) Viscero-sensory neurones, from the viscera, through the 

 dorsal root, to termination in the intermediate zone of the 

 gray matter. 



A new era in the investigation of cranial nerves was inau- 

 gurated by Strong ('95) when he made the application of these 

 principles to larval amphibians. Strong found that it is prac- 

 ticable to recognize certain distinct classes of fibres or 



