22O The Spinal Cord 



the leg. 5 (Cowers.) If there be a serious flaw in any part of the 

 chain the reflex does not work, and if there be a break in the fibres 

 descending from the brain we cannot control it. 



The reflexes are controlled by an inhibitory impulse descending 

 by the antero-lateral columns of the cord (pyramidal tracts). When 

 these columns are diseased the afferent impulse awakens in the grey 

 matter a motor wave of disproportionate vigour. The reflex is then 

 spoken of as ' exaggerated. ' Thus there is exaggerated knee-jerk when 

 the pressure of angular curvature interferes with the antero-lateral 

 columns of the cord. 



Tetany, muscular spasms, limited and general convulsions exagger- 

 ated spinal reflexes are met with so frequently in young children 

 because the control-fibres of the pyramidal tracts are late in acquiring 

 due functional activity. 



A reflex being lost, this question arises : 'Is there disease of the 

 postero-external column with which the sensory roots are associated, 

 as in locomotor ataxy ; or in the grey matter, as in antero-polio- 

 myelitis which had occurred in infantile or diphtheritic paralysis ; 

 or is there some degeneration affecting the anterior nerve-roots ? ' 

 On account of the disturbance in the grey matter, exaggeration of a 

 reflex often precedes its abolition. 



Special reflexes. When the skin in the pubic region of the thigh 

 is stimulated, an impulse is conveyed by the ilio-inguinal nerve to the 

 lumbar enlargement, and thence by the genital branch of the genito- 

 crural to the cremaster, which, contracting, draws up the tesficle. This 

 is the cremaster reflex ; it is generally we. 11 marked in childhood. Its 

 absence in the adult does not necessarily imply disease. The * centre ' 

 for this reflex is in the lumbar enlargement. 



Other superficial reflexes are the plantar, gluteal, abdominal, epi- 

 gastric, and scapular. 



It is unnecessary to describe each in detail, but one may say briefly 

 that by irritating sensory nerve filaments in any one of these regions 

 a gentle motor influence is duly passed out to the subjacent muscles 

 provided that the reflex chain be in working order. In the case of the 

 scapular reflex, irritation of the skin between the shoulder-blades sends 

 a quiver through the teres major. The reflex centre for the scapular 

 muscles is in the lower part of the cervical enlargement. When the 

 cerebral control is lost, or the segment of the cord within an indivi- 

 dual circle is excited by disease, the reflex is ' exaggerated.' 



By trying one reflex after the other on each side of the body, and 

 duly comparing them, the condition of the cord in almost its entire 

 length can be ascertained. 



The deep or tendon-reflexes are obtained by irritating the sensory 

 nerves of the muscles themselves. The muscle must first be placed 

 in a condition of moderate tension and then smartly struck or over- 

 stretched. 



