$i6 THE CENTRAL NERVOUS SYSTEM 



column. The anterior nerve-roots are perfectly healthy. The grey 

 matter of the cord at least, in the earlier stages of the disease 

 is unaffected. The weak link in the chain is the afferent path. 

 Where the presence of the knee-jerk is doubtful, it is necessary to search 

 for the most favourable position of the limb for eliciting it before deter- 

 mining 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. 911). In 

 anterior poliomyelitis (p. 876) 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 transection 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 the transection. The position of the centres 

 in the cord for the various reflex movements is shown in Fig. 366. 



3. The Origination of Impulses in the Spinal Cord (Automatism). 

 An action known to be caused or conditioned by afferent impulses 

 is called a reflex action. There is some evidence that the reflex 

 centres are continually in a state of activity, and are not simply 

 roused to activity by the arrival of the afferent impulses, which 

 discharge the reflex. Their condition, when not discharging, seems 

 to represent a state of balance' between excitatory and inhibitory 

 influences, a so-called ' innervation equilibrium,' which can be up- 

 set in one direction or the other, according to the intensity of the 

 antagonistic influences. A physiological action is termed auto- 

 matic when it depends upon a nervous outflow which seems to be 

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

 dent 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. 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 de- 

 monstrated 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 extended, until the controversy as to the boun- 

 daries between the two seems not unlikely to be ended by the ab- 

 sorption of the automatic in the reflex. And as we seem almost 

 driven to conclude 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 



