144 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



Spinal Reflexes in the Mammals. — Experiments upon the lower 

 mammals show that co-ordinated reflex movements may be ob- 

 tained from the cord after severance of its connections with the 

 brain. Sherrington* has described a simple operation by which the 

 head may be removed from an anesthetized cat and the animal be 

 kept alive for a number of hours. Stimulation of the skin in such 

 an animal calls forth numerous definite reflexes, such as flexion or 

 extension of the legs, the scratching movements of the hind legs, 

 stretching movements, etc. Or the spinal cord may be severed in 

 the thoracic region, below the origin of the phrenic nerves, and the 

 animal, with care, can be kept alive for months or years. In such 

 an animal reflex movements of the hind legs or tail may be ob- 

 tained readily from slight sensory stimulation of the skin. The 

 knee-jerk and similar so-called deep reflexes are also retained. But 

 it is evident that these movements are not so complete nor so 

 distinctly adapted to a useful end as in the frog. The muscles of 

 the body supplied by the isolated part of the cord retain, however, 

 a normal irritability and exhibit no wasting. In man, on the 

 contrary, it is stated that after complete section of the cord the deep 

 reflexes, such as the knee-jerk, as well as the skin reflexes, are very 

 quickly lost. The muscles undergo wasting and soon lose their 

 irritability, t The monkeys exhibit in this respect a condition that 

 is somewhat intermediate between that of the dog and man. It 

 seems evident from these facts that in the lower animals, like the 

 frog, a much greater degree of independent activity is exhibited by 

 the cord than in the more highly developed animals. According to 

 the degree of development, the control of the muscles is assumed 

 more and more by the higher portions of the nervous system, and 

 the spinal cord becomes less important as a series of reflex centers, 

 its functions being more dependent upon its connections with the 

 higher centers. 



Dependence of Co-ordinated Reflexes upon the Excitation 

 of the Normal Sensory Endings, — It is an interesting fact that 

 when a nerve tnmk is stimulated directly in a reflex frog — the 

 sciatic nerve, for instance — the reflex movements are disorderly 

 and quite unlike those obtained by stimulating the skin. It is said 

 that if the skin be loosened and the nerve twigs arising from it be 

 stimulated, an operation that is quite possible in the frog, the re- 

 sponse is again a disorderly reflex, whereas the same fibers stimu- 

 lated through the skin give an orderly, co-ordinated movement. 

 The difference in response in these cases is probably not due to any 

 peculiarity in the nature of the sensory impulses originating in the 

 nerve endings of the skin, but more likely to a difference in their 

 strength ard arrangement. When one stimulates a sensory nerve 

 * Sherring;ton, "Journal of Physiology," 38, 375, 1909. 

 t See Collier, " Brain," 1904, p. 38. 



