STRUCTURE AND PHYSIOLOGY OF ANIMALS 



29 



tissue is specially characteristic of parts in which rapid movement 

 IS necessary. 



The principal elements of nervous tissue are nerve-cells and 

 nerve-fibres. 



Nerve-cells (Fig. 21) vary greatly in form ; they are relatively 



FiQ. 19--^™-^*™t^d m^^^^ /, substance of fibre ; n, nucleus ; p, unaltered protoplasm in 



the neighbourhood ot the nucleus. (From Huxley's Lessons in PhysMoJy.) 



large cells with large nuclei and one or several processes, one of 

 which is always continuous with a nerve fibre. 



The nerve-fibres (Fig. 22), which are to be looked upon as greatly 

 produced processes of nerve-cells, are arranged for the most part 

 in strands which are termed nerves. The fibres themselves vary 

 greatly in structure in different classes of animals. In the higher 

 animals the most characteristic form of nerve-fibre is that which is 

 termed the medullated nerve-fibre. In this there is a central 

 cylinder — the axis-cylinder or neuraxis (A, ax) — which is the 



ml 





i-^sH 







Fig. 20. — striated muscle. A^ part of a muscular fibre of a Fi'og ; B, portion of striated muscle 

 teased out to sliow separation into fibrillje. (From Huxley's Lessons in Physiology.) 

 b, if, g, transverse bands and striaj ; ii, iiiiulci. 



essential part of the fibi'e and is made up of numerous extremely 

 fine primitive Jibrillce ; this is surrounded by a layer of a white 

 glistening material — the white siibstancc of Schwunn oi' medullary 

 sheath (mcd), enclosed in turn in a very delicate membrane — the 

 neurilemma (neur). 



The blood, the lymjDh, and other similar fluids in the body of an 

 animal may be looked upon as liquid tissues, having certain cells 



