126 PROFESSOR MARSHALL. 



use of the investigations of others, so far as known to me, as well as 

 of my own published in this Journal and elsewhere. 1 



Historical Sketch. — The older writers relied exclusively on 

 anatomical evidence in dealing with the problems before us, and 

 their determinations were rather of the nature of guesses than logical 

 endeavours to grapple seriously with the difficulties encountered. 

 Moreover, in the great majority of cases their judgment was influenced 

 in a very prejudicial manner by preconceived ideas on the morpho- 

 logical constitution of the skull. 



Inasmuch as these older theories are all based on the same 

 arguments, and differ from one another only in points of minor 

 importance, it will be sufficient to take one of them and examine it 

 critically. For this purpose I select the theory advanced by Stieda, 

 the most recent, indeed the only recent, advocate of the views in 

 question. 



Stieda, 2 in attempting to solve the problem of the segmental value 

 of the cranial nerves, commences by stating that as he accepts Oken's 

 theory that the skull consists of three vertebrae, the number of pairs 

 of segmental cranial nerves must necessarily be two ; viz., a pair 

 leaving the skull between the first and second skull-vertebree on either 

 side, and a pair emerging between the second and third skull- vertebrae, 

 the nerves passing out between the skull and the first cervical 

 vertebra being universally considered, when present, the first pair of 

 spinal nerves. 



Having in this very summary manner determined the number of 

 segmental cranial nerves, Stieda proceeds to divide the nerves actually 

 present into two groups in accordance with this determination. He 

 first rejects the nerves of special sensation, i.e. the olfactory, optic, 

 and auditory, on the ground that embryology shows than to be really 

 parts of the brain, and therefore not directly comparable with the other 

 nerves. 



Concerning the remaining nine pairs of nerves still left for 



consideration, he holds that the most reliable evidence is afforded 



by the fact that in certain groups of animals some of these nerves do 



not arise independently from the brain, but are represented by branches of 



other nerves. 



1 A list of the works consulted is given at the end of this paper. 



s Stieda, Studien iiber das centrals, Ncrvensystem der Wirbelthiere, Leipzig, 1870. SeparaU 

 Abdruck aus der Zeitschrift fii/r wissenschaftliche Zoologie, Bd. xx,, p, 166, seq. 



