318 PHYSIOLOGY CHAP. 



no strict relation between strength of stimulus and extent and 

 duration of reflex. According to Sherrington, the reflex arc some- 

 times behaves like cardiac muscle, which responds to stimuli by 

 maximal contractions or not at all (supra). In any case it is 

 certain that the internal conditions of the reflex arc have more 

 influence upon the degree of the reaction than the strength of 

 external stimulus. 



The seat of stimulation is an important factor in determining 

 the character of the reflex movements. The reflexes evoked by 

 stimulating the viscera are different from those excited by 

 cutaneous stimulation ; these, again, vary greatly according to the 

 point stimulated. This fact is easily demonstrated on the spinal 

 decerebrated frog. The constancy of the various reflex reactions 



/obtained on exciting different points of the frog's skin (by bits of 

 paper saturated with acidulated water) tends to show the existence 

 of a functional mechanism in the cord in which the character of 

 the reflexes is determined by the spatial position of the spot at 

 which the excitation arises. This is true, not only for cutaneous 

 sensations, but also for those which originate from the sensory 

 nerves of the muscles, tendons, and joints, and serve to identify 

 the position of the limbs in consciousness. 



Lastly, the character of the reflexes depends to a great extent 

 on the intrinsic conditions of the cord, i.e. the state of excitability 

 and conductivity of the spinal centres. Reflex excitability can be 

 raised or lowered by the action of specific toxic substances ; it is 

 depressed or abolished by anaesthetics, particularly with chloro- 

 form : reinforced by convulsants, especially strychnine. Various 

 diseases that involve the spinal cord produce a rise or fall in reflex 

 excitability, like poisons, so that the reflexes are exaggerated or 

 abolished. 



The functional condition of the spinal centres is chiefly 

 dependent on the circulation and the respiration. A spinal frog 

 in which the circulation is arrested by tying the heart, or the 

 . blood is replaced by an isotonic salt solution, reacts to cutaneous 

 stimulation for about half an hour if the temperature is low ; for 

 a shorter time with a higher temperature. In the spinal rabbit, 

 according to Sherrington, the reflexes do not last more than a 

 minute after the arrest of circulation at the normal temperature. 

 But if the animal had previously been cooled, the reflexes may 

 persist much longer. Both anaemia and asphyxia, before they 

 abolish the reflexes, cause temporary exaggeration, shown by a rise 

 of blood pressure, contraction of the bladder, erection of hairs, and 

 convulsive movements. 



In addition to these variations of reflex excitability and con- 

 ductivity in the spinal centres, which are produced by coarse 

 alterations in their physiological conditions, we must take into 

 consideration the other more delicate changes in the state of the 



