IV. OF RANA PIPIENS. 33 



to the mass of calcareous crystals occupying the vestibular cavity, and to the semi- 

 circular canals. 



VII. Vagus. (Plate I. Fig. 1, vn.; Plate II. Fig. 2, vn.) This is the last of 

 the pairs of nerves escaping through the cranial walls ; it arises from the dorsal 

 surface of the medulla oblongata a little behind its lateral portion, by three or 

 four slightly separated roots. The most posterior of these is attached somewhat 

 farther towards the median line on the motor or lower surface than the others, 

 and may therefore be compared to a motor root of a common spinal nerve ; if this 

 be identical with any separate nerve in the higher animals, it is with the acces- 

 sory of Willis, though it has not that peculiar origin, by numerous roots extended 

 for some distance along the medulla, as in higher Reptiles ; for example, in the 

 Tortoise.* Reasons for believing in this identity will be found in the description 

 of the ultimate distribution of the nerve. The trunk of the vagus makes its exit 

 from the cranial cavity through a hole in front of the occipital condyles, which, 

 when examining the cranium alone, might from its position be mistaken for the 

 anterior condyloid foramen. Immediately after leaving the cranial cavity it be- 

 comes involved in its ganglion, the largest of the whole series of ganglia connected 

 with the cranio-vertebral series of nerves. All the root fibres probably pass 

 through the ganglion. A filament of the sympathetic nerve unites the vagal and 

 hypoglossal nerves with each other, and between the two is a ganglion, the first of 

 the sympathetic series. The vagal trunks are three in number. 



A. The first and smallest of the three is given off from the upper portion of the 

 ganglion (Plate I. Fig. 1 and Plate II. Fig. 2, 6), is directed upwards till it reaches 

 the skin just above the tympanum, as described by Volkman, and is ultimately dis- 

 tributed to the integuments between the tympanic membranes and the eyes, and on 

 the scapular region from above downwards as far as the level of the lower edge of 

 the tympanum. This nerve is supposed b'y Muller to be the remnant of the ner- 

 vus lateralis of the tadpoles, a view which does not seem to be confirmed, since they 

 have the nervus lateralis in addition, and the nerve in question is not a branch of the 

 lateralis, but is derived directly from the ganglion. A more probable supposition 

 is to regai'd it as representing the dorsal branch of a common spinal nerve, a branch 

 which is repeated for every pair of nerves along the back ; and here we have ad- 

 ditional evidence in favor of the identity of the cranial and vertebral series of nerves. 



B. (Plate I. Fig. 1,#.) This trunk, after passing outwards one or two lines, di- 

 vides into two branches, one of which is directed forwards, and, curving around the 

 posterior surface of the vestibule, joins the branch from the trigeminus, with which 

 it forms the so-called facial nerve. The second branch is directed a little back- 

 wards, descends along the sides of the neck, where a minute filament is given to the 

 mucous membrane of the oesophagus, passes over the body of the hyoid bone, giving 

 filaments to the mucous membrane of the floor of the mouth, then along the upper 

 surface of the genio-hyoid muscle on the median line till it reaches the symphysis 

 of the lower jaw, where it enters the base of the tongue and is distributed to the 

 mucous membrane alone. When the tongue is retracted, this nerve is thrown into 



* See the figures of Bojanus in his Anatome Test. Europ., Plate XXI. Figs. 87 - 92. 



