Chap. VII.] 



THE NERVOUS SYSTEM. 



91 



iris (the termination of the motor nerve), which by its contrac- 

 tion narrows tlie pupil. Hence arises the well-lino wn phe- 

 nomenon of the contraction of the pupil when light falls upon 

 the eye. 



Or, again, food passing into the upper part of the intestine 

 stimulates the sensory nerves there. The impulse passes to the 

 spinal cord, is reflected from this centre toward the periphery, 

 and passing along the motor nerve stimulates to contraction the 

 appropriate muscular mechanism which causes a flow of bile 

 into the intestine. 



N.C. 



M.O. 



Fig. 74. — Reflex Arc (schematic). — 5.0., sensory organ; S.N., sensory neurone ; 

 iV.C, nerve centre; M.N., motor neurone; M.O., motor organ. 



Also, stimulation of taste fibres in the mouth causes a reflex 

 secretion of the salivary glands. Innumerable examples of this 

 kind might be given. Indeed, since physical life has been well 



Fig. 75. — Reflex Arc, as it is approximately in Man. — 1, Nerve terminal, 

 or sensory epithelium ; 2, dendrone of sensory neurone ; 3, cell-body in dorsal root 

 ganglion; 4, axone of sensory neurone; 5, dendrone of motor neurone ; 6, cell-body 

 in ventral horn; 7, axone of motor neurone; 8, end organ — muscle-cell, gland- 

 cell, ett. 



