914 REFLEX ACTIONS. [BOOK m. 



drawn up and thrust out again. And an exaggeration of the 

 ' knee-jerk ' or other 'tendon reflexes ' is a very common symptom in 

 certain spinal diseases. It is rarely if ever that reflex movements 

 of a really complicated character are observed. Moreover clinical 

 experience shews that in man, when a portion of the cord is 

 isolated, reflex actions carried out by means of that portion so 

 far from being exaggerated are much more commonly exceeding 

 feeble or absent altogether. In the cases in which the physio- 

 logical continuity of the lower with the upper part of the cord 

 has been broken by disease, by some growth invading the 

 nervous structures 6r by some changes of the nervous structures 

 themselves, we may attempt to explain the absence from the 

 lower part of coordinate reflex activity, such as is seen in the 

 lower animals, as due to the disease not only affecting the powers 

 of the actually diseased part, but influencing the whole cord 

 below, and either by inhibition, of which we shall speak presently, 

 or in some other way depressing its functions. But the same 

 absence of complex reflex movements is also often observed in 

 cases in which the cord has been severed by accident, and indeed, 

 though accidental injuries to the human cord generally produce 

 more profound and extensive mischief than that which results 

 in animals from skilful experimental interference, clinical ex- 

 perience tends, on the whole, to support the view that in man 

 the more complete subordination of the spinal cord to the brain 

 has led to the dying out of the complex reflex actions which 

 are so conspicuous in the lower animals. This however cannot 

 be regarded as distinctly proved. 



When we come to study voluntary movements we shall see 

 reason to think that in man, as in the lower animals, the will in 

 carrying out these movements makes use of complex nervous 

 mechanisms situated in the spinal cord, nervous mechanisms into 

 the working of which, as urged above, afferent impulses enter 

 largely; and it seems improbable that these spinal mechanisms 

 should be capable of being thrown into action by the will only. 

 In the act of walking for instance it is highly probable that the 

 movements of the legs are the direct results of the action of nervous 

 mechanisms in the lumbar cord brought into play by the will, 

 being thus, in an indirect manner only, the products of volitional 

 impulses ; and even in man, though clinical experience only affords 

 us instances of this machinery working apart from the brain in a 

 damaged condition and under unfavourable circumstances so that 

 the resemblance of the movements observed to the complete 

 act of walking is but feeble, still it seems similarly probable 

 that under more favourable circumstances the lumbar cord separ- 

 ated from the brain might as part of a reflex act carry out the 

 movements in a more complete and coordinate manner. 



592. We have dwelt above chiefly on reflex actions, in which 

 the efferent impulses cause contractions of skeletal muscles since 



