1 88 NERVE-CENTRES. [CH. XVII. 



Waller tests the excitability of nerve by the amount of current of action 

 it gives rise to ; not by the amount of contraction in the muscle to which it 

 leads. He finds that the effect of carbonic acid in large doses is to depress 

 the activity of nerve ; but after the gas is removed, there is greatly in- 

 creased activity. Ether acts similarly ; but with chloroform there is no 

 recovery. Small doses of carbonic acid increase the action-currents, and 

 Waller considers that the staircase effect in muscle (p. 124), and the similar 

 progressive increase noted in the action-currents of nerve as the result of 

 repeated stimulation is due to the evolution of this gas during activity. 



There can be no doubt that the existence of the electrical variation is a.s a 

 rule the index of the excitatory alteration in a nerve. In the isolated nerve 

 it is in fact the only change that can be detected. But in the present state 

 of our knowledge we are not justified in assuming that it gives an absolutely 

 faithful record. The electrical variation can be detected in a nerve for many 

 days after its removal from the body. Although the electrical change is a 

 concomitant of the real excitatory process, the former may be therefore 

 perceptible when other evidence of the existence of the latter fails. Moreover, 

 Gotch and Burch have obtained further evidence of the dissociation of the 

 electrical response from the excitatory process. In the frog's sciatic nerve, 

 it is possible with two stimuli in rapid succession to obtain only one electrical 

 response near the seat of excitation which has been cooled, while two such 

 responses occur in a more peripheral warmer region. 



CHAPTER XVII. 



NERVE-CENTRES. 



THE nerve-centres consist of the brain and spinal cord ; they 

 are characterised by containing nerve-cells, from which the nerve- 

 fibres of the nerves originate. Small collections of nerve-cells 

 are found also in portions of the peripheral nervous system, where 

 they are called ganglia. The spinal ganglia on the posterior 

 roots of the spinal nerves, and the sympathetic ganglia are 

 instances of these. 



The general arrangement of the cerebro-spinal axis is given in 

 the accompanying diagram. The nerves which take origin from 

 the brain are called cranial nerves; there are twelve pairs of 

 these; some of them, like the olfactory, optic, and auditory 

 nerves, are nerves of special sense ; others supply the region of 

 the head with motor and sensory fibres. One pair (the tenth) 

 called the pneumogastric or vagus nerves are mainly distributed 

 to the viscera of the thorax and abdomen, and a part of another 

 pair (the eleventh), called the spinal accessory nerves, unites with 

 the vagus prior to such distribution. We shall in our subsequent 

 study of the heart, lungs, stomach and other organs have frequently 

 to allude to these nerves. The first two pairs of cranial nerves (the 



