138 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



arrangement. Whether or not the reactions of the nervous system 

 in such cases are accompanied by any degree of consciousness can 

 not be proved or disproved, but the assumption of such an accom- 

 paniment does not seem necessary to explain the reaction. 



Spinal Reflexes in the Mammals. Experiments upon the lower 

 mammals, such as the dog, show that co-ordinated reflex move- 

 ments may be obtained from the lower portion of the cord after 

 severance of its connections with the brain. The spinal cord 

 may be severed, for instance, in the thoracic region and the animal 

 be kept alive and in good condition for an indefinite period. In 

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

 obtained 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 purposeful 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.* 

 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 func- 

 tions 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 trunk 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 are 

 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 and arrangement. When one stimulates a sensory nerve 

 trunk directly, the ulnar nerve at the elbow in ourselves, for in- 

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



