THE REFLEX ACTIONS OF THE SPINAL COKD. 573 



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 pro- 

 duces a start, a bright light makes the eye wink and may cause the person 

 to sneeze (the greater coordination manifest in this act being due to the 

 fact that the complex respiratory mechanism is brought into play ( 334), 

 and reflex movement 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 move- 

 ments, such as have been described above as occurring in frogs and mam- 

 mals after experimental interference, but rather mere exaggerations of the 

 simpler reflex movements witnessed when the nervous system is intact. In 

 cases of paraplegia (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 sensation 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 drawn up and thrust out again. And an exaggera- 

 tion of the " knee-jerk " or other " tendon reflexes " is a very common symp- 

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

 of a really complicated character are observed. Moreover, clinical experi- 

 ence shows that in man, when a portion of the cord is isolated, reflex 

 actions carried out by means of that portion, so far from being exagger- 

 ated, are much more commonly exceedingly feeble or absent altogether. In 

 the cases in which the physiological continuity of the lower with the upper 

 part of the cord has been broken by disease, by some growth invading the 

 nervous structures, or by some changes of the nervous structures themselves, 

 we may attempt to explain the absence from the lower part or coordinate 

 reflex activity, such as is seen in the lower animal, 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 pres- 

 ently, 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 



