314 PHYSIOLOGY 



ment would be impossible in the central nervous system of vertebrates, since impukes 

 must of necessity pass through the cell body on their way from the termination of one 

 axon to the beginning of the next. In the spinal root ganglion however, most of the 

 cells lie on the surface. In the rabbit Steinach exposed a posterior root ganglion, 

 separating it from all its vascular supply, but leaving its nervous attachments intact. 

 The wound was opened every day for the next few days and an instrument passed 

 under the ganglion so as to divide any newly forming vessels. As a result of the 

 deprivation of blood-supply the ganglion- cells died. But Steinach found that nerve 

 impulses were still conducted perfectly well through the ganglion at a time when 

 microscopic examination showed a complete atrophy of all cells. It is therefore only 

 in virtue of the fact that the nerve-cell is the seat of the nucleus, and therefore of the 

 assimilative functions of the neuron, that any pre-eminent importance can be ascribed 

 to it in the building up of a reactive nervous system. 



Prominent among the functions with which the nerve cell has been 

 endowed is that of automaticity of action in the absence of stimulus other 

 than that supplied by its own metabolism or by the fluids which bathe it. 

 A priori there is no reason to deny to the neuron a property which is pos- 

 sessed by other cells of the body, such as the muscular cells of the heart, and 

 is a fundamental quality of undifferentiated protoplasm. The purpose 

 however for which these cells have been evolved and differentiated is 

 that of reaction, of adapting the organism to changes in its environment, 

 and it is doubtful whether, in this differentiation, it has retained any auto- 

 matic properties whatsoever. In the absence of any afferent impulse the 

 whole central nervous system would probably be inert. In a frog retaining 

 only the spinal cord Hering divided all the posterior roots. The frog 

 remained flaccid and motionless. Injection of strychnine was powerless 

 to evoke the usual tetanic spasms. In such a strychninised frog however, 

 it was necessary only to open the wound and touch one of the divided 

 posterior roots to throw the whole body into convulsions. As shown by 

 Sherrington and Mott, division of all the afferent nerves coming from the 

 upper limb in monkey or man entirely abolishes all such contractions of the 

 limb, as are usually affected through the intermediation of the cerebral 

 cortex. Cutting off the major portion of the afferent impulses to the respira- 

 tory centre does not, it is true, abolish all respiratory discharges, but converts 

 the rhythmic respirations into a series of inspiratory spasms which are 

 repeated at long intervals and are entirely inadequate for the proper aeration 

 of the blood. According to Sherrington a repetition on the mammal of 

 Hering' s experiment does not lead to the same results, since a spasmodic 

 discharge is produced from the isolated spinal cord as a result of asphyxia. 

 But it is doubtful whether in this case there was not some continuous excita- 

 tion of the cord going on, as a result of the closure of the demarcation current 

 in the cut ends of the posterior roots by the body fluids. It is possible that 

 the neurons possess some automatic power, i.e. some power of initiating 

 nervous piOftMMg, as a result of changes in the fluids surrounding them. 

 Tliis automat ieity however is not a prominent feature of the nervous system, 

 which has l,een evoked as a purely reactive mechanism to the afferent 

 impulses resulting from the material changes continually taking place in the 

 'n\ iroimieiit of the animal. 



