8i2 A MANUAL OF PHYSIOLOGY 



to search for the most favourable position of the limb for eliciting 

 it before determining that it is absent. The patient may be 

 made to clasp his hands tightly at the moment of the tap to 

 reinforce the jerk (p. 807). In anterior poliomyelitis (p. 775) the 

 afferent link is intact, but the other two are broken, and the 

 reflexes also disappear. Certain lesions which partially cut off 

 the spinal cord from the higher centres without affecting the 

 integrity of the spinal reflex arcs increase the strength of reflex 

 movements and the facility with which they are called forth. In 

 primary spastic paraplegia (a paralysis of the legs and the lower 

 portion of the body), which is associated with degenerative changes 

 in the lateral columns, the deep reflexes are all exaggerated. But, 

 according to the best authorities, a lesion amounting to total tran- 

 section of the cord in man abolishes all reflexes below the lesion. 

 In the monkey the knee-jerk may be tried for in vain for weeks after 

 section of the cord in the middle of the thoracic region, whereas in 

 the rabbit it can be obtained ten to fifteen minutes after thetran- 

 section. The position of the centres in the cord for the various 

 reflex movements is shown in Fig. 338. 



3. The Origination of Impulses in the Spinal Cord 

 (Automatism). A physiological action is termed automatic 

 when it depends upon a nervous outflow which seems to be 

 spontaneous, in the sense that it is not brought about by any 

 evident reflex mechanism, or, in other words, is not discharged 

 by afferent .impulses falling into the centre where it arises, 

 although it may be determined by substances in the blood. An 

 action known to be caused or conditioned by such afferent 

 impulses is called a reflex action. Automatic actions being 

 thus denned in a negative manner by the defect of a quality, 

 there is always a possibility that some day or other it may be 

 demonstrated that any given action which at present seems 

 automatic in its origin depends on afferent impulses hitherto 

 unnoticed. As a matter of fact, the supposed proofs of spinal 

 automatism have in more than one case vanished with the 

 advance of knowledge, and as the domain of purely automatic 

 action has been narrowed, that of reflex action has ex- 

 tended, until the controversy as to the boundaries between the 

 two seems not unlikely to be ended by the absorption of the 

 automatic in the reflex. And as we seem almost driven to con- 

 clude that from the anatomical standpoint the nervous system 

 is essentially a vast collection of looped conducting paths, each 

 with an afferent portion, an efferent portion, and connections 

 between them formed by the end arborizations of the axons and 

 their collaterals, the dendrites and the cell-bodies, so it may 

 be that no strict physiological automatism really exists either 

 in cord or brain, that every form of physiological activity 

 muscular movement, secretion, intellectual labour, conscious- 

 ness itself would cease if all afferent impulses were cut off 

 from the nervous centres. Assuredly no neuron is entirely 



