STRUCTURE AND PHYSIOLOGY OF ANIMALS 



27 



tissue is specially characteristic of parts in which rapid movement 

 is necessary. 



The principal elements of nervous tissue arc ncrcc cells and 

 nerve fibre*. 



(Fig. 18) vary greatly in form ; they are relatively large 



Fii;. lii. Non striated muscle cell; /. sub-it.-nn-e of fibre ; )>. nucleus ; />. unaltered protoplasm in 

 the neighbourhood of the nucleus. (From Huxley's ..S,VOK.S in Physiology.) 



cells with large nuclei, and one or several processes produced into 

 nerve fibres. 



The in'i'Ci' nl'cs (Fig. 19), which are to be looked upon as greatly 

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

 in strands which are termed no-res. The fibres themselves vary 



i/ 



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 meduUated nerve-fibre. In this there is a central cylinder 



the axis-cylinder* QT ne urn-vis (A, an} which is the essential part 



B 



-7? 



-6 

 -cl 



6 



Km. 17. Striated muscle. A, part of a muscular fibre of a Frog; B, portion of stri;ited muscle 

 teased out to show separation into fibrillas. (From Huxley's X-.s.<o,i.s i,i PliiitMouii.) 



of the fibre, and is made up of numerous extremely fine primitive 

 fibrillcK ; this is surrounded by a layer of a white glistening 

 material the wldt? substance of ftchwann or medullary slienti! 

 (med), enclosed in turn in a very delicate membrane the 

 'nc in'U<: 'ii i ma (ncur). 



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

 nnini:il may be looked upon as liquid tissues, having certain cells 



