396 PRINCIPLES OF GENERAL PHYSIOLOGY 



and we can see, by taking such a case as that of the superior cervical ganglion, that 

 the non-medullated fibres are isolated from each other. Aledullated fibres leave 

 the spinal cord and proceed to this ganglion, where they form junctions, 

 "synapses," with another set of neurones, whose fibres are not medullated. In 

 this ganglion, therefore, non-medullated fibres of various functions are mixed 

 together, but isolated functionally. These various kinds of fibres are vaso- 

 constrictors, dilators for the iris and nerves to the different secretory glands. 



Although the medullary sheath is formed by cells independent of the neurone 

 itself, it must have an intimate connection with it, since, on cutting the axi-. 

 cylinder free from the nucleated cell body to which it belongs, the medullary 

 sheath undergoes degeneration along with the axis cylinder. When nerve fibres 

 regenerate by growing out again from the cells, the remains of the old sheath^ 

 seem to act as guides for the new fibres, which grow down into them. 



It has been suggested that this myclin sheath may serve as a source of nutrition to the 

 fibre. It is very doubtful, as we have seen, whether there is any metabolism in the nerve 

 fibre to require food. 



After treatment with fixing reagents, the contents of the axis cylinders appear 

 as a number of ^laments, " nntro-fibrils," as they have been called. There is no 

 actual proof of their presence in the living state, and they are, in all probability, 

 produced by the action of the reagents used. Mott (1912) finds no indication of 

 their presence in the living nerve cell. 



On the contrary, Carlson (1911) has brought forward evidence to show that 

 the axis cylinder has the properties of a liquid, just as protoplasm in general. 

 Certain animals, such as the slug^ exhibit great changes in their length ; the nerve 

 fibres must be stretched in the long form of the slug, since they are not folded up 

 in the shortened condition of the animal. Now, Carlson finds that, if the pedal 

 nerves are excited close to the pedal ganglia with the animal artificially stretched 

 and again when unstretched, the time which elapses before the foot muscle 

 contracts is greater in the former case. It is important to note that the degree of 

 stretching was not such as to affect the excitability of the nerve, which was tested 

 by using submaximal stimuli, and the same height of contraction found in both 

 cases. If the nerve is stretched too much the muscle enters into contraction, 

 and also the extent of the stretching was about that of the normal crawling 

 movements. What are we to conclude from the result obtained ? It is evident 

 that the nerve itself was actually stretched and not merely uncoiled, since the 

 latter would have had no influence on the time of conduction. It is difficult to 

 understand how any substance but one having the characters of a liquid could be 

 increased in length without showing any result beyond an increase in time of 

 conduction proportional to the increase in length. Carlson, in fact, shows that the 

 rate of conduction is unaltered. Since increase in length implies decrease in 

 diameter, the result indicates that the rate of conduction is independent of the 

 sectional area of the axis cylinder. 



<ii>thlin (1913) finds that both the axis cylinder and the medullary sheath are doubly 

 refracting. That of the former is of the kind shown by fibres consisting chiefly of protein, 

 such as muscle fibres and connective tissue, but comparatively slight. That of the latter is 

 similar to that of liquid crystals of the glycero-phosphatides. It was also shown by polarisa- 

 tion methods that the apparently non-medullated fibres of many invertebrates have a sheath 

 of glycero-phosphatides and that the Crustacea show a gradually increasing development of 

 medullary sheath from the lower to the higher members of the class. 



The Mature of the Nerre Impulse. It may be useful to collect together the 

 evidence obtained so far on this question. 



That it is a reversible, physico-chemical process, not associated with loss of 

 material on account of metabolic reactions, is indicated by the following fa 



Incapability of fatigue under normal conditions. 



Absence of formation of heat. 



Absence of decrement in wave. 



Low temperature coefficient of rate of conduction. 



No conclusive evidence of metabolism of any kind. 



On the other hand, the existence of fatigue in the absence of oxygen points to 



