REFLEX PATHS. 



391 



More complicated spinal reflexes are those of defecation, mic- 

 turition, parturition, vasomotor reflexes, cardio-accelerator reflexes, 

 etc. The impulses traverse at least three neurones in these reflexes; 

 because all efferent white rami communicantes terminate in some 

 ganglion proximal to the organ supplied. As an example, trace 

 a defecation reflex. 



Defecation Reflex. The rectum is supplied by the third and 

 fourth sacral nerves and by branches of the inferior mesenteric 

 and hypogastric plexuses. Irritation of the sensory endings in 

 the mucous membrane is caused, normally, by the presence of 

 feces. The impulses caused thereby run to the special defecation 

 center in the lumbar enlargement of the spinal cord, either by 



Fig. 112. Diagram of a simple reflex arc. (After Brubaker.) 



i. Sentient surface. 2. Afferent nerve. 3. Emissive or motor cell. 4. Efferent nerve- 



5. Muscle. 



way of the sacral nerves or through the sympathetic plexuses, 

 the ganglionated cord, and the rami communicantes to the lumbar 

 nerves, through the posterior roots of which they reach the center 

 in the cord. From the defecation center the impulses pursue 

 two courses: (a) They descend through the third and fourth 

 sacral nerves and cause inhibition in the circular fibers of the 

 rectum and contraction of the longitudinal muscle, (b) This 

 action is immediately followed by impulses which pursue the 

 sympathetic course, through the anterior roots of the lumbar 

 nerves, the rami communicantes, the ganglionated cord, and the 

 inferior mesenteric and hypogastric plexuses, to the rectum. 

 They cause, in succession from above downward, contraction 

 of the circular muscle of the rectum. The two series of impulses 



