844 THE SPINAL CORD. 



Even when the stimulus, instead of being restricted to a narrow sensory- 

 area or path, affects simultaneously large surfaces or wide channels, finally 

 the whole skin, the motor discharge evoked is still confined wholly 

 to harmoniously acting spinal units. Thus the spinal frog placed in 

 water warm enough to stimulate the general skin, swims forward with 

 balanced and symmetrical and synchronously bilateral strokes. The 

 co-ordination of the action of the musculature of each limb, and that of 

 the hind-limbs together, and even of fore-limbs with hind-limbs, is good ; 

 the aquatic locomotion is surprisingly efficient in view of such total loss 

 of all the cranial senses. 



These considerations clearly indicate that some set of spinal 

 sense organs forms the chief basis of the mechanism which ensures such 

 elemental muscular co-ordination as that under discussion. The fact that 

 it is fairly preserved even in the skinned spinal preparation, indicates 

 that the spinal sense in question does not depend on cutaneous sense 

 organs. Its sense organs are probably those in the musculo-articular 

 apparatus, since it seems hardly possible that the co-ordination is to so 

 large an extent an innate quality of the motor unit groups themselves. 



The integrity of the ivhole cord is not required to ensure co-ordination. 

 Short isolated portions are enough. In Limulus four pairs of the 

 abdominal ganglia isolated in a group can regulate the respiratory move- 

 ments of the eight appendages co-ordinately, in total absence of the 

 whole remainder of the nervous system. 1 In spring the three upper 

 spinal segments, isolated from the rest of the head and brain with their 

 connected skin and muscles, execute the frog's sexual " clasp." In the 

 cat and monkey, with spinal transection low in the sacral region, wagging 

 of the tail from side to side by alternate co-ordinate contraction of its 

 lateral musculature, is evoked from the local skin. In short, to employ 

 the metaphor of resistances, in the reflex spinal organ the resistances 

 are so disposed as to ensure synthesis not of a discord but always of a 

 harmony of movement. 



The influence which the condition of one part of the cord exerts upon 

 others is seen in the effect which the initial posture of a limb can be demon- 

 strated to exert upon the character of the reflex movement elicited from the 

 limb. From the skin of the back of the proximal part of the thigh in the frog 

 and dog, the reflex response in the limb is flexion at the hip, if the resting 

 posture of the limb at the time of application of the skin stimulus be one 

 of extension, e.g. if the limb be hanging down. If, however, the resting 

 posture of the limb be one of flexion at the time of application of the skin 

 stimulus, the reflex movement of response is often an extension of the thigh 

 at hip. The difference of the response may be due to the different condition of 

 the musculo-articular sense organs in the two different initial postures, their 

 difference reacting upon the excitability of the antagonistic motor mechanisms 

 of the cord. The experiment is significant of the important part assignable to 

 subconscious muscular sense reactions in regulating the activity of the skeletal 

 musculature. It suggests an explanation for the alternating character of the 

 movements executed by spinal reflexes. 



Phasic variation in the reflex activity of the cord. — Although 

 the relative invariability of the movement elicited by repetitions of a 

 particular stimulus is a striking character of spinal reflexes in the mammal, 

 and lends to them a machine-like quality of regularity, curious variety 

 of result occurs, when they are examined in the same individual 



1 Ida Hyde (Loeb's laboratory), Joum. Morphol., Boston, 1894, vol. ix. 



