190 FUNCTIONS OF THE BRAIN. 



ditions do not explain the rhythmic repetition of the reactions in one case and the 

 absence of rhythm in the other. It indicates that there is some "open and shut" 

 mechanism that can be reached and set into action via g.o., gc 1 , h; or via g., gc. 2 , 

 gc 1 , h.; but not via th.t. gc. 1 , h. or via any other way. 



II. THE CROSSED THORACIC REFLEXES. 



The experiments show that : 



1. Gentle stimulation (temperature) on one side of the thorax first causes 

 aimless movements of the opposite legs, followed by aimless movements of the 

 legs on the same side. If the stimulation is increased, the legs of the same side 

 make coordinated movements that tend to thrust the stimulating object away; 

 and finally the opposite side may join in the coordinated movements. 



2. If the nerve collar is cut on one side between, say, the second and third 

 neuromeres, stimulation of the cut side produces the same results as before, 

 except that the legs back of the cut, on the same side, do not make purposeful 

 "thrusting away" movements, while those cephalad to the cut continue to do so, 

 if the sides of the thorax cephalad to the cut are stimulated. 



3. If all the free thoracic commissures are cut, the crossed reflexes cease. 

 These results show : a. that certain coordinated purposeful movements on one 



side of the thorax are controlled by the corresponding side of the forebrain, in 

 all probability by the corresponding hemisphere; b. that the path of the direct 

 crossed impulses is through the thoracic commissures, and c. that the coordination 

 of purposeful movements on one side of the thorax with those on the other is 

 accomplished via the commissures at the base of the forebrain. 



The simpler relations of these reflex paths are shown in connection with the 

 gustatory tracts in Fig. 114. It will be noted that while stimulation of the 

 temperature organs at th.t. produce a few simple movements of several legs of the 

 opposite side, stimulation of the taste cell at g.o. produces a continued rhythmic 

 discharge into the muscles at the base of one leg on the same side, with the result 

 that first a leg flexor, then a leg extensor muscle contracts, followed by the con- 

 traction of the four plastro coxals, pi. ex., and then by the five coxo-tergals, cx.t. 



III. THE CROSSED AND UNCROSSED ABDOMINO-THORACIC REFLEXES. 



When the ventral margin of the abdomen is gently stimulated, the legs on 

 the opposite side of the thorax are aimlessly raised, followed by a start or spasm 

 of the legs on the same side. 



Numerous experiments show that a. the abdominal temperature impulses 

 may cross on entering the cord, passing cephalad to the opposite crus. (Fig. 114, 

 ab. t 2 ) ; b. that the greater number of impulses pass up the same side of the cord 

 they enter, and that all these impulses cross to the opposite side through the com- 

 missures of the vagus neuromeres, ab.t 3 ', c. that they do not cross in the free tho- 

 racic commissures. 



