56 VETERINARY PHYSIOLOGY 



which they could not have done had there been such 

 structures. 



These cells give off at least one process, which continues 

 for some distance, as the axon. Frequently other processes 

 are given off, which form a branching system of dendrites. 

 The axons end in dendrites, so that all the processes are 

 essentially the same. These processes appear to be fibrillated, 

 and in fixed specimens the tibrillse may be traced through 

 the protoplasm of the cells, but this appearance is probably 

 an artefact due to the action of the fixing agents employed. 

 In many cases the dendrites show little buds or gemmules 

 upon their course, and, according to some observers, it is 

 through these that one neuron is brought into definite 

 relationship at one time with one set of neurons, and at 

 another with other adjacent neurons. There is also some 

 evidence that the dendrites as a whole may expand and 

 contract, and thus become connected with those of adjacent 

 neurons. 



2. Axon. — The axon process, as it passes away from the 

 cell, becomes a Nerve Fibre, and acquires one or two cover- 

 ings. 



(1) A thin transparent membrane, the primitive sheath or 

 neurolemma, is present in all peripheral nerves. Between it 

 and the axis cylinder there are a number of nuclei 

 surrounded by a small quantity of protoplasm, the nerve 

 corpuscles. Fibres with this sheath alone have a grey colour, 

 and they may be called grey or non-inedullated fibres. 

 They are abundant in the visceral nerves. This sheath 

 is absent from the nerve fibres of the central nervous 

 system. 



(2) A thick white sheath — the medullary sheath or white 

 sheath of Schwann — which gives the white colour to most 

 of the nerves of the body appears somewhat late in the 

 development of many nerve fibres. It lies between the 

 primitive sheath with its nerve corpuscles and the axon. It 

 is not continuous, but is interrupted at regular intervals by 

 constrictions of the neurolemma at the nodes of Ranvier 

 (fig. 20). It is composed of a sponge- work or felt-work of 



