42 ON THE NERVOUS SYSTEM IT. 



cavity, and are only separated after development has advanced to a certain stage, 

 we have a strong ground for the hypothesis, that all the bones of the face which 

 are developed in the walls of the primitive cavity of the mouth which they sur- 

 round, are in their anatomical and physiological relations splanchnic, connected 

 either with digestion or respiration, rather than parts of the endo-skeleton of 

 animal life. 



The conclusions which have been drawn from the statements made above are as 

 follows : that in Frogs the vagus comprises the glosso-pharyngeal and accessory 

 nerves ; that the trigeminus comprises the facial, the abducens, and in the Salaman- 

 ders the patheticus and portions of the motor communis ; that other evidence 

 sustains the hypothesis, that the whole of the motor communis is a dependence of 

 the trigeminus ; if to these we add the hypoglossus (which in Frogs is exceptionally 

 a spinal nerve), we shall have three pairs of cranial nerves, each having all the 

 characters of a common spinal nerve, namely, motor and sensitive roots and a 

 ganglion ; that there are no nerves to indicate a fourth vertebra, unless the special 

 sense nerves are considered; if these are admitted as indications, then we must 

 presuppose either two pairs of nerves to each vertebra, or the existence of six 

 vertebrae, which is a larger number than can be accounted for on an osteological 

 basis. The functions and mode of development of the special sense nerves we 

 have taken as affording sufficient grounds for considering them as of a peculiar 

 order, and not to be classified with common spinal nerves. 



SECTION VI. SPINAL NERVES. 



I. Hypoglossus. (Plate I. Fig. 1,1.) Another remarkable feature in the nervous 

 system of Frogs, and one which has been noticed in other tailless Batrachians also, 

 is the fact that the hypoglossal nerve (the ninth pair of Willis, and the eleventh 

 of Soemmering), is not, as in most Vertebrates, one of the cranial nerves, but the 

 first of the dorsal or true spinal series. Its origin is at the extreme portion of the 

 medulla oblongata, just in front of the contraction of the chord which precedes 

 the brachial enlargement. In reference to the determination of its true affinities, 

 it is of consequence to notice the fact, that it is provided with two kinds of roots, 

 of which the anterior or motor are the most numerous, consisting of a bundle of 

 filaments attached a little nearer to the median line than the corresponding ones 

 from the vagus. The posterior or sensitive root is quite small and scarcely per- 

 ceptible ; it does not appear to have been noticed, except by Volkman. Its minute- 

 ness is such, that in removing the membranes, unless especial care be taken, it will 

 be torn away with them, and therefore easily escape detection. The presence of this 

 dorsal or sensitive root serves to identify it with a true spinal nerve ; and the iden- 

 tification is rendered complete by the existence of a minute ganglion near its 

 junction with the motor root. The two kinds of root fibres, just after escaping 

 from the spinal canal between the first and second vertebrae, unite and form a 

 single trunk, which almost always is provided with a small sacculated appendage 

 filled with calcareous crystals. The trunk of the nerve descends along the sides 



