IV. OF RANA PIPIENS. 39 



which is met with in Man, except the spinal accessory. It docs not appear, that 

 in one and the same animal the number of cranial nerves is reduced so low 

 as six pairs, except in the lowest of Fishes, namely, in the genera Amphloxus, 

 Myxine, Lepidosiren, and Bfhilostonm.* 



While the cranial nerves of Frogs are less simple than in the genera just men- 

 tioned, they are more so than in any Mammals, Birds, or Eeptiles, except the allied 

 Batrachians ; and they are especially interesting as showing the gi*eatest reduction 

 met with in any air-breathing Vertebrates. By tracing out the distribution of some 

 of the branches of the trigeminus and vagus, we are enabled to identify them with 

 the separate pairs of nerves into which they are resolved in the ascending series. 

 The branch from the vagus distributed to the tongue is easily identified with 

 the glosso-pharyngeal, and the branch to the muscles of the shoulder with the 

 spinal accessory ; thus we have a demonstration of the fact, that the vagus is made 

 up by the union of three nerves, which in Man are so many independent pairs. 

 In like manner, some of the branches of the trigeminus are identified with separate 

 pairs of nerves in the higher Vertebrates, as, for example, the facial, and the motor 

 externus; in Salamanders, the patheticus, and some (as the rectus superior, pp. 

 26, 27), if not all, of the branches of the motor communis. Thus the theory which 

 makes the typical number of cranial nerves (independently of the special sense 

 nerves) three pairs, namely, trigeminus, vagus, and hypoglossus, becomes highly 

 probable. 



As regards the special sense nerves, they have some peculiarities which seem to 

 indicate that they are of a different order from all the rest of the nerves connected 

 with the cerebro-spinal axis. Their peculiarities relate, — 1st, to the fact that 

 they-are connected with special organs of sense ; 2d, to their development. 



According to the ablest embryologists of tlie present day, common spinal nerves, 

 and the same is true of the crauio-spinal nerves, do not shoot out from the respec- 

 tive portions of the chord to which they are attached, but are developed in the 

 tissues where they are respectively found, and this independently of the central 

 axis, just as the bloodvessels of the germinating membrane are developed inde- 

 pendently of the central organ of circulation. This view, or something very near 

 it, was maintained by Gall, and more recently has been presented by Bischoff, 

 Kolliker, and others. 



Such is not the mode of the development of the special sense nerves. According 

 to Reichert and the more recent observers, the special sense nerves, as well as a por- 

 tion of the organ of sense itself, are the result, in the first instance, of a kind of 

 hernia, or protrusion of a portion of the embryonic cerebral vesicles, which are 

 evolved and prolonged outwards, until they meet an involution of the common in- 

 tegument, and from the two results the organ of sense. The sense nerve at first 

 seems to be formed by the contraction of the evolved tube, forming a hollow pedicle, 

 but is subsequently filled up with nerve fibres. 



One other difi"erence may be referred to in connection Avith the special sense 



* For the anatomy of the brain of Myxinoid Fishes, sec Johannes MuUer, Berlin Trans., 1837. 



