122 INTRODUCTION TO GENERAL PHYSIOLOGY 



clear that there is not protoplasmic continuity, and that the two 

 neurones are independent of one another, as far as their nutrition is 

 concerned. The usual cell membrane intervenes, and is here called 

 the " synaptic membrane," having some special properties. The 

 physiological process must clearly be transmitted through this 

 membrane or no reflex would result. This transmission is perhaps 

 an electrical effect in which ions change places, but the process is 

 still obscure in its details. 



In the greater number of reflexes there are one or more inter- 

 mediate or " association " neurones, as in the arc E, A, D of Fig. 6. 

 As the nervous system increases in complexity, we find that the 

 number and length of these association neurones increases, so that, 

 while in the earthworm they only extend to one or two adjoining 

 segments, in the higher vertebrates they reach to the cerebral 

 hemispheres themselves. These higher parts are formed entirely 

 of such neurones. 



A word may be said here as to the nature of the protoplasm 

 composing the substance of the neurone. If examined, while still 

 alive, under dark ground illumination, the protoplasm of the body 

 of the cell is seen to be filled with numerous granules in Brownian 

 movement, which appear to congregate together for a time in 

 various parts of the cell, so that larger and more brilliant particles 

 are formed locally. These granules are said to show signs of the 

 possession of a fatty sheath which stains with methylene blue. 

 The movements show that they are suspended in liquid, so that the 

 structure of the neurone is in general similar to that of other 

 protoplasm. There are also reasons for believing that the nerve 

 fibre process, "axon," as it is called, is also of a liquid nature. 



It may be asked, what is the function of the nucleated part of 

 the neurone? It may possibly act as a kind of relay, adding 

 energy to a nerve impulse which has become weakened by passing 

 through the synapse. But this is by no means certain. It has 

 been shown that reflexes can take place after these parts of the 

 neurones have been removed, as is possible in some animals. It is 

 clear, however, that the continued life of the neurone depends on 

 this nucleated part, as would be expected from the statements 

 made in our first chapter with regard to the functions of the 

 nucleus. If a part of the neurone is separated from the "trophic" 

 influence of the nucleus, it dies, disintegrates, and ceases to carry 

 impulses. 



Comprehension of the general principles on which the central 

 nervous system is constructed may be assisted by a short account 

 of its evolution. In the lowest multicellular animals, the sponges, 

 there are no structures comparable to nerves, although they possess 

 effectors in the form of muscle cells under the layer of amoeboid 



