550 PHYSIOLOGY CHAP. 



since this partially destroys the nervous tissue and abolishes its 

 excitability. 



The mechanical excitability of the cortex in the depths of the 

 cruciate sulcus is no accidental or exceptional fact ; it can invari- 

 ably be demonstrated in all dogs in which the electrical excitability 

 of the superficial cortical centres is well preserved. 



All previous observers found the superficial cortex of the 

 sigmoid gyrus inexcitable to mechanical stimuli. In very 

 exceptional cases only Hitzig (1877) observed movements of one 

 limb during the removal of the corresponding centre. It is prob- 

 able that the normal mechanical excitability of the motor centres 

 of the cortex is easily exhausted, long before the electrical ex- 

 citability, by mere exposure of the surface to the air. The cortex 

 in the cruciate sulcus, on the contrary, keeps its excitability 

 longer. 



The action of chemical stimuli on the cerebral cortex produces 

 different effects. Landois (1891) found that on sprinkling the 

 motor zone of the dog with various constituents of urine clonic 

 convulsions set in after a long latent period, which lasted a longer 

 or shorter time and were more or less generalised all over the 

 body. Maxwell (1906) observed that these symptoms of excitation 

 are due, not to stimulation of the ganglion cells of the cortex, 

 but to osmotic or chemical excitation of the nerve-fibres in the 

 subjacent white matter, which, as we know from other experiments 

 on nerve (see p. 219), react to these stimuli. 



But in another series of experiments he found that certain 

 chemical substances, like creatine, act directly upon the elements 

 of the cortical grey matter. In fact, the application of creatine, 

 solid or strongly concentrated, to the cortex is followed after 

 rather a long latent period by clonic and tonic contractions, 

 while the injection of creatine solutions into the depth of the 

 white matter, and steeping the motor nerve trunks in saturated 

 solution of the same substance, fails to evoke signs of reaction. 



Baglioni and Magnini (1909) worked out the effects of different 

 chemical substances (acetic, citric, carbolic, glyceric acids, urea, 

 sodium chloride, sodium sulphate, strychnine, picrotoxin, and 

 curare) when applied to the excitable zones of the cerebral cortex 

 of the dog. After exposing the motor zone and determining the 

 threshold of the faradic excitability of a given centre, they applied 

 the chemical substance, and employed the threshold of faradic 

 excitability to ascertain the stimulating or depressing action of 

 the c'hemical substance employed, independent of the direct motor 

 reactions which it produced. 



From their results they were able to divide the chemical 

 substances which affect the centres in the motor zone into two 

 distinct groups. 



(a) The first includes the acids employed, and glucose, urea, 



