CELLULAR DIFFERENTIATION. 55 



appearance of separate mesoderm from which the whole muscular system 

 is derived. 



80. Nervous Tissue: its Functions. The nervous tissues 

 are in close relation on the one hand to the sensory epithelium 

 and on the other to the muscular tissue. Through the former 

 they receive the stimuli from the outside world; by means of 

 their connection with the latter they are enabled to effect 

 responses. The reception of stimuli, the transmission of the 

 results of stimulation, and the initiation of appropriate re- 

 sponses constitute the fundamental work of nervous tissue 

 (Fig. 28, A, D}. In some of the lower Metazoa the same cell 

 may do all these tasks. 



81. Structure. The principal elements of nervous tissue 

 are the nerve-cells (ganglion-cells} and nerve- fibres. The 

 cells, which are the centres of nervous activity, are usually 

 large with conspicuous nuclei. The fibres are, in their essen- 

 tial parts merely outgrowths of the ganglion-cells. These out- 

 growths are of two sorts : the dendron, which is a much, and 

 irregularly, branched structure ; and the axon, or nervous fibre 

 proper. Each cell may have one or more processes arising 

 from it. These fibres may pass just as they arise from the 

 cells, without special structural modification, to their connec- 

 tions. Such are called non-medullated or gray fibres. There 

 are usually however one or more protective sheaths formed 

 about this essential axis: (i) the medullary sheath, consisting 

 of a framework filled with a fatty material, surrounded by 

 (2) Schwann's sheath, a homogeneous sheath with occasional 

 nuclei along its course (Fig. 28, D}. Fibres possessing the 

 medullary sheath are called medullated or white fibres. 



Questions on Fig. 28. What are the principal points of contrast 

 between the plain and the cross-striate muscular fibres? Enumerate the 

 principal regions of the nerve cell figured. How does it differ from a 

 typical cell in form? What are the principal parts of the nerve-fibre (D) ? 

 What are the supposed functions of these various portions? Why is it 

 necessary for nerve cells to be in connection with other kinds of cells? 

 What are the differences between the contracted and uncontracted muscle 

 fibre (B and C) ? What is meant by a neurone? 



