30 



ZOOLOGY 



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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 



'. 



FiO. 19. Non-striated muscle-cell. /, substance of fibre ; n, nucleus ; p, unaltered protoplasm 

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



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 



essential part 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 

 Schwann or medullary 

 sheath (med), enclosed 

 in turn in a very deli- 

 cate membrane the 

 neurolemma (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 

 the corpuscles disseminated through a liquid plasma, which 

 takes the place of the ground-substance of the connective tissues. 

 In a large proportion of cases such corpuscles are similar to 

 Amoebae in their form and movements (amoeboid corpuscles, leuco- 

 cytes). In the blood of Vertebrates leucocytes occur along with 

 coloured corpuscles of definite shape containing the red-colouring 

 matter (hcemoglobin) of the blood. The leucocytes are able, like 

 Amoebae, to ingest solid particles, and under certain conditions a 



B 



_ 6 





. 



FIG. 20. Striated muscle. A, part of a muscular fibre of 

 a Frog ; R, portion of striated muscle teased out to 

 show separation into fibrilhe. (From Huxley's Lessons 

 in Physiology.) b, d, g, transverse bands and stria? ; 

 n, nuclei. 



