CENTERS IN THE SPINAL CORD 587 



vented (cf. page 576). Likewise, one finds in pigeons with the spinal cord 

 cut that every stimulus applied to a toe causes a reflex impulse to be sent to 

 the flexor muscles of the same leg and at the same time a reflex impulse to 

 the extensor muscles of the other leg (Singer). Similar, though not identical, 

 results have been obtained in the dog (Freusberg). 



Unquestionably these reflexes are components in the mechanism of locomo- 

 tion, and hence we may say that the locomotor movements observed in decapi- 

 tated animals are conditioned primarily upon this coincidence of flexor and 

 extensor reflexes from the cord. In dogs and monkeys having the spinal cord 

 cut at the posterior end of the thorax, Heger has recently observed fairly 

 well coordinated movements of the hind parts. The animals were even able 

 to walk and run with the help of the hind legs, although, as was to be 

 expected, not with the same precision as normal animals. 



The same mechanism is probably operative also in the uninjured nervous 

 system, the motor impulse given out by the brain being apportioned auto- 

 matically and in the proper order to the different motor cells down the cord. 



We have still to mention the tendon reflexes. When a person crosses one 

 leg over the other, allowing the foot to hang in an unrestrained position, then 

 strikes the patellar tendon of this leg a sharp blow, the extensor cruris muscle 

 contracts suddenly giving the so-called " knee jerk." Similar contractions are 

 obtained by stimulation of other tendons, and by mechanical stimulation of the 

 joints and of the periosteum. They are wanting in tabes i. e., they are wanting 

 when certain afferent conducting pathways are disabled. 



In view of this latter circumstance, it is natural to suppose, as Erb first 

 pointed out, that these contractions are pure reflexes. But various considera- 

 tions, especially the short latent period of the knee jerk, made that explanation 

 very difficult, and Westphal, the discoverer of such phenomena, now takes the 

 view that the contractions are induced by the direct effect of the mechanical 

 shock upon the muscles, but only in case the muscle has its normal tonus. This 

 condition, as we know (cf. page 581), depends upon a normal state of the 

 afferent nerves. 



Evidence that the tendon reflexes are dependent upon the central nervous 

 system is found in the fact that their intensity varies directly with the general 

 functional condition of the central system. Thus they are weakened by fatigue, 

 hunger, and the like, but are augmented by rest, food, etc. (Lombard). 



B. INFLUENCE OF THE SPINAL CORD ON THE VEGETATIVE FUNCTIONS 



The above-mentioned observations on the behavior of the dog with a short- 

 ened spinal cord (page 583) call for some revision of our views as to the 

 influence of the cord on the vegetative functions ; for several of these functions 

 which from previous observations had been regarded as totally dependent upon 

 the spinal cord were there seen to be dependent upon peripheral nervous 

 mechanisms. The urinary and sexual organs, for example, as well as the 

 rectum, remained perfectly functional, even when the entire lower part of 

 the spinal cord was destroyed. But these parts are controlled to some extent 

 also by centers in the central system, as can be shown conclusively by stimu- 

 lation of the appropriate nerves. These centers are located mainly in the 

 lumbar region of the cord. 



