CHAP, i.] THE SPINAL CORD. 915 



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 produce contractions of 

 cither muscles, as well as other effects, such as secretion for in- 

 stance. 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 ( 549) with the general problems of nutrition, we stated 

 that an orderly nutrition appears to be in some way dependent 

 on nervous influences. Many of these nervous influences appear 

 to issue from the spinal cord, either as parts of a reflex act, or as 

 the outcome of some automatic processes. When in a dog the 

 lumbar cord is wholly separated from the rest of the cord by 

 section, the nutrition of the hind limbs, and the general health 

 of the animal may, with care, be maintained in a very satisfactory 

 condition; but if that small separated piece of the cord be des- 

 troyed death inevitably ensues before long, in spite of every care 

 and precaution, being brought about apparently by the disordered 

 nutrition of the hind limbs and other parts supplied by nerves 

 coming from the lumbar cord. 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. The scarcity of well marked reflex actions 

 mentioned above as characteristic of such cases, may perhaps be 

 due to the fact that these disorders of nutrition prevent the 

 patient living long enough for the separated cord to recover the 

 functions which properly belong to it. 



593. 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 deve- 

 lopement 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 movements, 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 afferent impulses. The micturition centre in the 

 mammal, which is also largely a reflex centre, may be easily in- 

 hibited 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, 



