THE FROG 83 



The ninth nerve, known as the glosso-pharyngeal, is a mixed nerve, 

 but mainly sensory, arising from the medulla behind the eighth by 

 roots in common with the tenth nerve. They leave the skull together 

 by the same foramen, the jugular foramen, situated on the outer side 

 of the exoccipital condyle, immediately before leaving which they 

 form a large jugular ganglion. On quitting this the ninth sends a 

 small branch forwards, which, as already noted, anastomoses with the 

 hyomandibular branch of the seventh nerve. The remaining branch 

 passes round the pharynx to the floor of the mouth and pursues a 

 characteristic wavy course below and inside the anterior cornu of 

 the hyoid cartilage. It supplies the mucous membrane of the floor 

 of the mouth and the tongue, in which it ends. 



The tenth is a large mixed nerve, called the vagus or pneumo- 

 gastric, and it arises and leaves the skull in the way indicated above. 

 It is unlike all the preceding cranial nerves in that it is distributed 

 to structures outside the head. After quitting the jugular foramen 

 it gives off a few twigs to the muscles of the back, and passes in the 

 wall of the pharynx backwards and downwards, dividing into four 

 main branches. The first, the recurrent laryngeal, loops round the 

 pulmo-cutaneous artery and runs inwards to the larynx. The 

 second or cardiac passes to the heart, the third or pulmonary to the 

 lungs, and the last or gastric breaks into two smaller branches going 

 to the stomach and other viscera. 



Spinal Nerves. 



There are in the adult ten pairs of spinal nerves, for 

 although others are present in the tadpole, they disappear during 

 the course of its development. Each nerve is mixed and arises 

 from the spinal cord by two quite distinct fairly equi-sized roots. 

 The dorsal root comes from the dorso-lateral aspect of the cord 

 and is related to the dorsal horn of grey matter within it. A 

 short distance from its origin it bears a swelling, the dorsal root 

 ganglion, and immediately after unites with the ventral root. The 

 ventral root is similarly related to the ventral horn of grey matter, 

 and leaving the ventro-lateral border of the cord runs, without any 

 ganglionic enlargement, to join the dorsal root. The common trunk 

 so formed is surrounded by a deposition of calcium carbonate forming 

 a conspicuous white patch, and passes out of the vertebral column 

 through the intervertebral foramen. It divides almost at once into 

 two unequal branches ; a small dorsal branch or ramus which runs 

 dorsally to the muscles, and a larger ventral ramus which is the main 

 nerve. 



The first spinal nerve is known as the Hypoglossal, and it leaves 

 the vertebral column by the intervertebral foramen between the first 



