. 



STRUCTURE AND PHYSIOLOGY OF ANIMALS 27 



tissue is specially characteristic charts in which rapid movement 

 is necessary. 



The principal elements of nervous tissue^re nerrc cells and 



nerve fibres. ^^B 



Nerve cells (Fig. 18) vary greatly in form ; the^^ffe relatively large 



K, It). Xon striated muscle cell ; /. substance of fibre ; n. nucleus ; p. unaltered protoplasm in 

 the neighbourhood of the nucleus. (From Huxley's Lessons in Physiology.) 

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

 nerve fibres. 



The nerve fibres (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 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 medullatcd nerve-fibre. In this there is a central cylinder 

 the H.i'ix-cifUndcr or ncn-mxis (A, an] which is the essential part 



B 







FIG. 17; Striated muscle. A, part of a muscular fibre of a Frog ; B, portion of striated muscle 

 teased out to show separation into fibrillaj. (From Huxley's Lessons in Physioloyi/.) 



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

 fibrillce : this is surrounded by a layer of a white glistening 

 material the white substance of Schumann or medullary sheath 

 {med), enclosed in turn in a veiy delicate membrane the 

 neurilemma (neur). 



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

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



