520 



MANUAL OF PHYSIOLOGY. 



nourished. This is not the case if the anterior root be divided, 

 but, on the contrary, that portion next the cord is well nourished, 

 while that going to join the posterior root is degenerated. 



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, 

 FIG. 207. 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 spe- 

 cial senses will be considered in future 

 chapters. 



Two cells from the for- 

 mer seen under a high 

 power, showing the fine 

 protoplasm here and there 

 retracted from the cell- 

 wall. 



III. THE MOTOR OCULI NERVE. 



The nerves of the third pair are the 

 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 cere- 

 bellar arteries. They pass into the orbits in two branches, and 

 are distributed 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 



