FROG : NERVOUS SYSTEM AND SENSE ORGANS 75 



membranous labyrinth of the ear. The ninth or glosso- 

 pharyngeal nerve arises from the side of the medulla behind 

 the eighth, immediately joins the tenth nerve, and passes 

 through the ganglion of the latter. It then bears a small 

 petrosal ganglion of its own, gives a branch to join the 

 hyomandibular nerve, and proceeds round the throat to 

 turn forward and run along the floor of the mouth, supply- 

 ing various structures there. The tenth or vagus nerve is 

 large and very important. It arises by several roots 

 adjoining the ninth, with which it is fused as far as the 

 jugular or vagus ganglion. It then turns backward and 

 downward round the throat and gives branches to the 

 larynx, heart, lung, and stomach. Through the branch 

 which runs to the heart that organ receives from the central 

 nervous system stimuli which raise or lower the strength 

 and frequency of its automatic beat. The impulses which 

 lower the beating arrive through the roots of the vagus from 

 the brain : those which raise it come through a branch of 

 the sympathetic system which joins the vagus (see below). 

 The cranial nerves do not, like the spinal nerves, arise 



each by a sensory arid a motor root, but it is 

 Functions of possible to distinguish among them a purely 

 Nerves. sensory series, a purely motor series, and a series 



of mixed function. The tenth, ninth, seventh, 

 and fifth nerves are mixed. They correspond to the dorsal 

 roots of spinal nerves with the addition of that part of the 

 ventral root (the autonomic part) which passes by the rami 

 communicantes to the sympathetic system. This part in 

 the cranial nerves passes direct to the alimentary, canal 

 and vascular system. Each of the mixed nerves retains its 

 dorsal root ganglion as a member of the series formed by the 

 vagus, glosso-pharyngeal, auditory, geniculate, and Gasserian 

 ganglia. The sixth, fourth, and third nerves are purely motor 

 and correspond to the ventral roots of spinal nerves, the 

 sixth being the ventral root of the seventh, the fourth that 

 of the fifth, and the third that of a nerve whose dorsal root 

 is contained in the ophthalmic nerve. The eighth is purely 

 sensory and represents part of a dorsal root. Its ganglion 

 is embedded in the labyrinth of the ear. The second and 

 first nerves are also purely sensory, but are not comparable 

 to the dorsal roots of spinal nerves. 



