522 



MANUAL OF PHYSIOLOGY. 



FIG. 207. 



It would thus appear that the trophic function of the ganglia 

 is restricted to the sensory nerves, while the nutrition of the 

 motor nerves is provided for by nervous centres situated higher 

 up. 



THE CRANIAL NERVES. 



The nerves which pass out through the foramina in the base of 

 the skull must be considered separately, 

 as the function of each of them shows 

 some peculiarity. Some are exclusively 

 nerves of special sense, and may be most 

 conveniently described when the special 

 sense organs are under consideration. 

 Some are simple, being purely motor in 

 function, while others are exceedingly 

 complex, containing many kinds of fibres. 

 They may be taken in the order of their 

 functional relationships, motor and mixed. 

 Those which relate to the special senses 

 willbe considered in future chapters. 



Two cells from the former 

 seen under a high power, 

 showing the fine protoplasm 

 here and there retracted 



from the cell wail. 



-.--,--,- m -*- ^ -.-,- 



III. THE MOTOR OCULI NERVE. 



The f fa ^ . fa 



chief motor nerves of the eyes. They arise 

 from the gray matter on the floor and roof 

 of the aqueduct of Sylvius, and pass out of the brain substance 

 near the pons from between the fibres of the peduncle, and then 

 run between the posterior cerebral and superior cerebellar 

 arteries. They pass into the orbits in two branches, and are dis- 

 tributed to the following orbital muscles: (1) elevator of the 

 eyelid ; (2) the superior, (3) inferior, and (4) internal recti; and 

 (5) the inferior oblique. They also contain fibres which carry 

 efferent impulses to the (1) circular muscle of the iris, and 

 to the (2) ciliary muscle. The latter branches reach the eye by 

 a short twig from the inferior oblique branch, which goes to the 

 ciliary ganglion, and thence enter the ciliary nerves. 



The action of the orbital muscles is, in the main, under the 

 control of the will, though they afford good examples of peculiar 



