39 2 . 



TRACING OF IMPULSES. 



thus open a way for the passage of fecal matter; and, then, force 

 it through the opening unless prevented by the voluntary contrac- 

 tion of the external sphincter. 



(2) Cerebral Reflexes. The simplest of these reflexes are 

 such as spasm of the muscles of mastication caused by a bad 

 tooth, in which both limbs of -the arc are formed by the trigeminal 

 nerve. Again, the facial expression of pain due to the same 

 cause. In this the impulses traverse the trigeminal nerve and 



Fig. 113. A more complicated spinal reflex arc, involving the fasciculi proprii. 

 (Brubaker after Kolliker.') 



Diagramjshowing the relation of the third neurone a, to the afferent neurone b, and to the 

 efferent neurones c, c, c. 



by the collaterals of its root-fibers reach the nucleus of the facial. 

 Through the facial they cause contraction of certain muscles of 

 expression. Facial spasm in tic douloureux is due to the same 

 reflex. The involuntary expansion of the nostrils upon the detec- 

 tion of a faint odor is due to an olfactory-facial reflex. The con- 

 nection of the terminal nucleus and cortical center of the olfactory 

 nerve with the genetic nucleus of the facial nerve is very much 



