38 



ON THE NERVOUS SYSTEM 



IV. 



twelve pairs in the one case are reduced to seven in the other. In the subjoined 

 tabular view of the cranial nerves, as they are presented in some of the dift'erent 

 members of the Vertebrate series, it is further seen that there exists a transition 

 from the more simple condition, in Amphioxus, Myxine, and Lepidosireii among 

 Fishes, and in Frogs among Reptiles, Avhere the nerves are less numerous, to the 

 most complex, in Mammals and Man ; that the trigeminus and vagus, as we 

 traverse the series, are, as it were, split up and dismembered, so as to make from 

 the smaller number in the one case the larger in the other. 



Table of the Number of Pairs of Cranial Nerves found in some of the Members of the Four Classes 



of Vertebrates. 



The instances in which the nerves are reduced to the lowest degree of numerical 

 simplicity are very few, in comparison with those in which the larger portion, if 

 not the whole series, of twelve exists. This simplicity is not, however, regulated by 

 the zoological position of the animal ; for in Frogs there exist but seven pairs of 

 cranial nerves, whilst in most Fishes eleven are found, — the whole series, in fact, 



* This enumeration of the nerves of Amphioxus deviates from that of Quatrefages, in his admirable 

 memoir on the anatomy of this interesting Vertebrate. By reference to the figures of Quatrefages, it 

 will be seen that the five pairs described by him may be easily reduced to three, if we regard the branches 

 I and m (Plate XI.) as forming but one nerve, and corresponding to the ascending and descending 

 branches of a common spinal nerve in the same animal, and the branches o and p (Figs. 4 and 5) as 

 accessory filaments to the nerve I. The branch m does not vary materially in its size and distribution 

 from the corresponding part in the pairs vvliich precede it. The great size of the branch / is at once ex- 

 plained when compared with the greater amount of surface which it supplies, and the more acute sensi- 

 bility which exists about the head. The branches 7, m, o, p, would therefore form but a single nerve, 

 which may be compared with the trigeminus ; and if to this we add the optic and olfactory, we have two 

 pairs of special sense nerves and one of cranio-spinal nerves. Rathke regards all the nerves as true 

 spinal ones, and the brain as wholly deficient. See Quatrefages, Annales des Sci. Nat., p. 197. 

 Oct., 1845. 



