S22 



THE SPINAL CORD. 



found. In the spinal rabbit, on the other hand, the crossed reflex from one hind- 

 limb to the other is often not an asymmetrical movement, but a symmetrical 

 flexion; this seems to stand in obvious relation to the hopping mode of 

 progression of the animal. 



3. The law of unequal intensity of bilateral reflexes. When the 

 excitation of a sensory nerve elicits reflex action involving both halves 

 of the body, and the action is unequal on the two sides, the side of 

 stronger contractions is always that homonymous with the seat of 

 application of the stimulus. 



This statement is in conformity with a number of instances. The following 

 are examples. When bilateral retraction of the abdomen is excited from the 

 skin of the chest, the contralateral retraction is much the less marked ; in the 

 bilateral protraction of the " whiskers " (cat, rabbit, dog) on excitation of the 



FIG. 356. Diagram of the different movements of the head and fore-limb (spinal 

 cat) caused by stimuli applied to the fore-paw and to the pinna respectively ; 

 the former evokes a movement of the head toward the stimulus with pro- 

 traction and lifting of the fore-foot, the latter a retraction of the fore-limb 

 and a twisting of the head toward the opposite side ("torticollis reflex"). 

 1, The pose of the spinal animal before the excitation ; 2, pose after stimula- 

 tion of left fore-paw ; 3, pose after stimulation of left pinna. 



skin of the face, the crossed movement is the less ample. An interesting 

 illustration, because it involves inhibitory as well as pressor influence, can be 

 demonstrated in the spinal cat or dog thus l : The animal resting comfortably 

 on its back, if one hind-paw be pressed, that leg will be flexed at hip, knee, 

 and ankle, and if the stimulus be strong, or the reflex excitability good, the 

 fellow hind-limb will be extended. If instead of one hind-paw both hind- 

 paws be pressed, both hind-limbs are simultaneously flexed, and there is no 

 trace of extension. The homonymous reflex is prepotent, therefore, and 

 inhibits the crossed reflex. 



But there are also a number of exceptions to this "law," among others, the 

 abduction of the tail from the side stimulated, already referred to. 



4. The fourth of Pfliiger's classical "laws" of spinal reflex action states 

 that with associated spinal reflex centres the irradiation spreads more 

 easily in the direction toward than in the direction away from the head. 



1 Sherrington, Phil. Tram., London, 1897. 



