IV. OF RANA PIPIEXS. 23 



Section IV. — Peripheral Portion of the Nervous System. 



The peripheral system, composed of the different nervous trunks, which it is here 

 proposed to describe somewhat in detail, is in Frogs comparatively simple ; far more 

 so than in any Mammals, Birds, or higher Reptiles, and more so even than in most 

 Fishes. This simplicity results in part from the reduction in number both of cra- 

 nial and spinal pairs of nerves ; in the former it seems as if by suppression of some 

 of them, but in reality it is by the union of two or more pairs which in other ani- 

 mals form so many separate nerves. Another condition in which its simplicity is 

 also manifest is in the absence of those extensive anastomoses and plexuses, which 

 render the whole system of the animals in which they exist so intricate. In con- 

 sequence of these peculiarities, the nervous system of these animals becomes a 

 matter very interesting to the physiologist and anatomist, enabling him to study 

 the whole in a form almost typical, perhaps as much so as a natural form ever 

 is, and at the same time throwing some rays of light on the subject of philo- 

 sophical anatomy. 



The number of pairs of nerves connected with the central axis and escaping 

 through the walls of the cranium or the vertebral column, does not exceed in 

 all seventeen, of which seven are given oif either by the cerebral masses or the 

 medulla oblongata,* and ten from the spinal chord. If we compai^e the number 

 of pairs of spinal nerves, as indicated by the number of vertebra? in the different 

 species of Vertebrates noticed in the tables of Cuvier, it will be found that in none, 

 a few Batrachians alone excepted, is it reduced so low as in Frogs. Accoi'ding to 

 Cuvier the following species present the smallest number of vertebraj in the classes 

 to which they respectively belong. 



Gibbon (Mammal), 31 Vertebrse. 



Hoopoe (Bird), 35 " 



Chelys anatomata (Reptile), ......... 38 " 



Oslracion triangularis (Fish), ......... 15 " 



Frogs (Cuvier, 9 pairs), .......... 10 " 



Pipa (Surinam Toad), 8 " 



"With regard to the cranial nerves, there exist among Fishes even, as will be 

 seen hereafter, only the genera Amphioxus, Bdelostoma, Myxine, and Lcpidostren 

 in which the number is known to be actually less, the first having but three 

 pairs, Lepidosiren five, and the others but six. Pctromyzon, according to MuUer, 

 has nine pairs, and according to Panizza, eight. In the larger portion of the 

 whole Vertebrate division, at least ten or twelve pairs may be made out, the ac- 

 cessory nerve being the one Avhich is most frequently deficient. Tlie following 

 table exhibits an enumeration of the cranial nerves of Man and Mammals con- 

 trasted with those of the Frog, showing at the same time what pairs are united 

 in the latter, so as to reduce the whole number from twelve to seven pairs. 



* " In the brain of the Frog only eight separate pairs are found, the facial, glosso-pharyngeal, acces- 

 sory of Willis, and hypoglossal exhibiting no distinct roots " ; " the hypoglossal is given olT by the first 

 pair of cervical nerves." Wagner, op. cil., p. 151. This statement, as will be seen, is not strictly appli- 

 cable to the present species. 



