THE. SEGMENTAL VALUE OP THE CRA.NIAL NERVES. 127 



Seasoning from these data, Stieda comes to the conclusion that the 

 component factors of his first cranial segmental nerve are the third 

 or oculomotor, the fourth or trochlear, the fifth or trigeminal, the 

 sixth or abducent, and the seventh or facial nerves ; and that of these 

 the third, fourth, sixth, and seventh nerves, and the motor root of 

 the fifth together represent the anterior or motor root, while the 

 sensory portion of the fifth nerve is the representative of the posterior 

 or sensory root. In support of these conclusions he adduces the 

 following arguments : — 



1. That the three eye-muscle nerves and the facial nerve may 

 sometimes be replaced by branches of the trigeminal, 1 and therefore 

 may be considered as belonging primarily to that nerve. 



2. That the three eye-muscle nerves, the facial nerve, and the portio 

 minor of the trigeminal behave with reference to their origin from the 

 brain like the anterior roots of the spinal nerves ; the portio major 

 of the trigeminal, on the contrary, like a posterior root : meaning by 

 this, the relations of the nerves in question to the nuclei of origin 

 within the substance of the brain. 



The second or posterior cranial segmental nerve he considers to be 

 made up of the ninth or glossopharyngeal, the tenth or vagus, the 

 anterior roots of the eleventh or spinal accessory, and the twelfth 

 or hypoglossal nerves; the ninth, tenth, and anterior roots of the 

 eleventh making up the posterior root, and the twelfth nerve represent- 

 ing the anterior or motor root, the main grounds of determination being 

 the same as those relied on in the case of the supposed first nerve. 



I have quoted Stieda at some length mainly in order to direct 

 attention to the nature of the evidence on which he attempts to solve 

 the question. The main points on which he relies are contained in 

 the passages I have italicised above, viz., (1) that the nerves of special 

 sense are contrasted with the other cranial nerves as being, properly 

 speaking, parts of the brain and not nerves in the strict sense of the 

 word', and (2) that in certain groiips of animals one or more of the 

 cranial nerves may lose their more usual independent character and 

 appear as, or be replaced by, branches of some other nerve; and 

 further, that this is to be taken as indicating that the nerves in 

 question were originally branches of this other nerve, and that their 

 independent origin from the brain, when it does occur, is a secondarily 

 acquired feat ure. 



All the cases in which this replacement is alleged to occur will be discussed later on in 

 this paper. 



