CHAP, i.] THE SPINAL CORD. 913 



is touched, may perhaps be regarded as the type of these move- 

 ments. A very common form of reflex action is that in which a 

 muscle or group of muscles is thrown into contraction by stimula- 

 tion of the overlying or neighbouring skin, as when the abdominal 

 muscles contract upon stroking the skin of the abdomen or the 

 testicle is retracted upon stroking the inside of the thigh. A 

 reflex movement may occur as the result of stimulation of an organ 

 of special sense, parts of the central nervous system other than 

 the spinal cord serving as the centre. A sound or a flash of 

 light readily produces a start, a bright light makes the eye wink 

 and may cause a person to- sneeze (the greater coordination 

 manifest in this act being due to the fact that the complex res- 

 piratory mechanism is brought into play, 391), and reflex move- 

 ments may result from a taste or smell. A special form of 

 reflex action, or at least an action resembling a reflex action, is 

 called forth by sharply striking certain tendons ; for instance 

 striking the tendon below the patella gives rise to a sudden 

 extension of the leg, known as the "knee-jerk"; but it will be 

 best to discuss these 'tendon reflexes' or 'muscle reflexes' as they 

 are called later on in another connection. 



On the whole the reflex movements carried out by the intact 

 nervous system of man are we repeat scanty and comparatively 

 simple ; but we are not justified in inferring from this that the 

 human spinal cord, left to itself, is incapable of doing more, that 

 owing to the predominant activity of the brain it has lost the 

 powers possessed by the spinal cord in the lower animals. For it 

 may be that the cord, when joined to the brain, is through various 

 influences proceeding from the latter in a different condition from 

 that in which it is when separated from the brain ; indeed we 

 have reason to think that this is so ; and we may here remark 

 that in the lower animals, as in man, the development of reflex 

 movements is difficult and uncertain in the presence of the brain. 



When we turn to the teaching of disease however, we again 

 find that reflex movements carried out by the cord or by parts of 

 the cord are, on the whole, scanty and simple. 



In some stages of certain diseases of the spinal cord extensive 

 reflex movements are witnessed ; but these are not purposeful 

 coordinated movements, such as have been described above as 

 occurring in frogs and mammals after experimental interference, 

 but rather mere exaggerations of the simpler reflex movements 

 witnessed when the nervous system is intact. In cases of para- 

 plegia (such being the term generally used when disease or injury 

 has cut off the cord, generally the lower part of the cord, from 

 the brain so that the will cannot bring about movements in, and 

 the mind derives no sensations from, the parts below the lesion, 

 the legs for instance), it sometimes happens that contact with the 

 bedclothes, or other external objects, sets up from time to time 

 rhythmically repeated movements, the legs being alternately 



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