132 



TEXT-BOOK OF PHYSIOLOGY. 



4. An efferent nerve, distributed to a responsive organ, as 



5. Muscle, gland, blood-vessel, etc. 



Such a combination of structures constitutes a reflex mechanism 

 or arc, the nerve portion of which is composed of but two neurons 

 an afferent and an efferent. An arc of this simplicity would of neces- 

 sity subserve but a simple movement. The majority of reflex activ- 

 ities, however, are extremely complex, and involve the cooperation 

 and coordination of a number of structures frequently situated at 

 distances more or less remote from one another. This implies that 

 a number of neurons are associated in function. The afferent 

 neurons are brought into relation with the dendrites of the efferent 

 neurons by the end-tufts of the collateral branches, which may extend 



for some distance up 

 A and -down the cord be- 



fore passing into the 

 various segments. 



For the excitation of 

 a reflex action it is es- 

 sential that the stimulus 

 applied to the sentient 

 surface be of an inten- 

 sity sufficient to develop 

 in the terminals of the 

 afferent nerve a series of 

 nerve impulses, which, 

 traveling inward, will be 

 distributed to and re- 

 ceived by the dendrites 



of the emissive or motor cell. With the reception of these impulses 

 there is apparently a disturbance of the equilibrium of its molecules, 

 a liberation of energy, and, in consequence, a transmission outward 

 of impulses through the efferent nerve to muscle, gland, or blood- 

 vessel, separately or collectively, with the production of muscular 

 contraction, glandular secretion, vascular dilatation or contraction, 

 etc. I The reflex actions take place, for the most part, through the 

 spinal cord and medulla oblongataJwhich, in virtue of their contained 

 centers, coordinate the various organs and tissues concerned in the 

 performance of the organic functions. The movements of mastica- 

 tion; the secretion of saliva; the muscular, glandular, and vascular 

 phenomena of gastric and mtestinaX ^digestion ; the vascular and 

 respiratory movements; the mechanism of micturition, etc., are illus- 

 trations of reflex activity. 



FIG. 58. DIAGRAM SHOWING STRUCTURES CON- 

 NECTED WITH REFLEX ACTIONS. A. Trans- 

 verse section of spinal cord with centers in the 

 anterior horn of the gray matter for muscles, 

 m, glands, g, and blood-vessels, b. ef.n. Efferent 

 nerves which convey nerve impulses to these 

 organs, s. Sensory surface, af.n. Afferent 

 nerve conveying nerve impulses to the centers 

 in the spinal cord. 



