40 ON THE NERVOUS SYSTEM IV. 



nerves ; namely, that in the animal series they are never seen combined with motor 

 roots, are never presented under the aspect of a true spinal or vertebral nerve. 

 It has been maintained by Blainville and others, that the motor nerves of the eye- 

 ball are the nerves, "of which the optic is the sensitive portion, and that they are 

 separated in consequence of the high degree of specialization of function. This 

 assumption is readily disproved by the fact, that, of these same motor nerves, the 

 pathetic, abducens, and some, if not all, of the branches of the motor communis, can 

 be shown to be dismemberments of the trigeminus. The opinion of Oken, which 

 considers the special sense nerves as appendages of the brain, is far more rational, 

 and has the merit of being more in accordance with facts. 



Cranial nerves may therefore be divided into two groups ; namely, into 



1. Special sense nerves : 2. Cranio-spinal nerves : 



I. Olfactory, A. Trigeminus, 



II. Optic, B. Vagus, 



III. Auditory ; C. Hypoglossus ; 



the second including the only true serial repetitions of spinal nerves. 



The determination of the typical number of cranial nerves becomes a matter of 

 importance, in consequence of its bearing upon the philosophical anatomy of the 

 skull. On the hypothesis that the cranium is made up of a series of vertebrae, 

 one of the first questions which is presented is as to the number which enter into 

 its composition. Throughout the vertebral column, as ordinarily understood, the 

 pairs of nerves equal the number of vertebral pieces. If the cranium is reducible 

 to the vertebral type, we might from analogy expect that there would be found a 

 series of nerves corresponding in number to the vertebrae of which the cranium is 

 made up. If we admit only such nerves as present the true spinal character to be 

 indicative of the number of vertebra?, that is, those which have motor and sensitive 

 roots, are provided with ganglia, and have a similar mode of development, then, 

 according to the analysis given above, there being three pairs of nerves conforming 

 to the spinal type, we should infer the existence of three vertebra. 



Anatomists, however, have not generally followed these indications, and it is a 

 singular fact, that, in establishing the number of cranial vertebrae, they have rested 

 their conclusions on such widely different foundations. Oken, in his latest pub- 

 lication, admits the existence of four vertebrae, based on the organs of sense and 

 the lower jaw, and which he designates as the " nose vertebra," the " eye vertebra," 

 the "ear vertebra," and the "jaw vertebra." Bojanus also recognizes four, and 

 substitutes the " tongue vertebra " for Oken's jaw vertebra. Agassiz admitted but a 

 single cranial vertebra, since the chorda dorsalis of the embryo did not extend be- 

 yond that portion of the base of the skull which corresponds with the basilar portion 

 of the occiput. Professor Owen, the most recent writer on the subject, who has inves- 

 tigated it very minutely, and has worked out his system with admirable skill, bases 

 his vertebral theory upon the principal subdivisions of the encephalon, from which 

 he deduces a " rhinencephalic," " prosencephalic," " mesencephalic," and " epen- 

 cephalic " vertebra. A larger number has been admitted by others, as Geoffroy St. 

 Hilaire, Carus, and Maclise, based mainly on an unsound determination of the 

 different osseous elements. 



