Chap, i.] THE SPINAL CORD. 707 



in which the efferent impulses cause contractions of skeletal 

 muscles since these are undoubtedly the most common and 

 the most prominent forms of reflex action ; but it must not be 

 forgotten that the efferent impulses of reflex origin may pro- 

 duce contractions of other muscles, as well as other effects, 

 such as secretion for instance. On several of these we have 

 dwelt, from time to time in previous parts of this work, and it 

 will be unnecessary to repeat them here. But it may be worth 

 while to point out that the spinal cord by serving as a reflex 

 centre for innumerable ties which correlate the nutritive or 

 metabolic activities of the several tissues to events taking place 

 in other parts of the body, plays a conspicuous part in securing 

 the welfare of the whole body. In dealing (§ 439) with the 

 general problems of nutrition, we stated that an orderly nutri- 

 tion appears to be in some way dependent on nervous influ- 

 ences. Many of these nervous influences appear to issue from 

 the spinal cord, either as parts of a reflex act, or as the out- 

 come of some automatic processes. In man, extensive injuries 

 to the spinal cord are followed by bed sores and other results 

 of impaired nutrition ; and indeed death is generally brought 

 about in this way, in cases of paraplegia caused by accidental 

 crushing or severance of the cord. 



§ 466. Inhibition of Reflex Action. The reflex actions of 

 the spinal cord, like other nervous actions, may be totally or 

 partially inhibited, that is to say may be arrested or hindered 

 in their development by impulses reaching the centre while it 

 is already in action. Thus if the body of a decapitated snake 

 be allowed to hang down, slow rhythmic pendulous move- 

 ments, which appear to be reflex in nature, soon make their 

 appearance, and these may be for a while arrested by slight 

 stimulation, as by gently stroking the tail. We have already 

 seen that the action of such nervous centres as the respiratory 

 and vaso-motor centres, which frequently at all events is of 

 a reflex nature, may be either inhibited or augmented by affer- 

 ent impulses. The micturition centre in the mammal, which is 

 also largely a reflex centre, may be easily inhibited by impulses 

 passing downward to the lumbar cord from the brain, or 

 upward along the sciatic nerves. In the case of dogs, whose 

 spinal cord has been divided in the thoracic region, micturition 

 set up as a reflex act by simple pressure on the abdomen or by 

 sponging the anus, is at once stopped by sharply pinching the 

 skin of the leg. And it is a matter of common experience that 

 in man micturition may be suddenly checked by an emotion or 

 other cerebral event. The erection centre in the lumbar cord, 

 also in large measure a reflex centre, is similarly susceptible of 

 being inhibited by impulses reaching it from various sources. 

 And indeed many similar instances of the inhibition of reflex 

 movements might readily be quoted. 



