62 



ZOOLOGY 



82. Origin of Muscle Tissue. — In those animals in which a true mesoderm is 

 wanting, the epithelial cells may develop, at their inner extremity, contractile 

 roots, either plain or striate (Hydra, Fig. 19, B). These cells may wholly lose 

 their epithelial quality and position and become entirely muscular. In the higher 

 animals this is very much modified by the early appearance of separate mesoderm 

 from which the whole muscular system is derived. 



83. 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 re- 

 sponses. The reception of stimuli, the transmission of the results 

 of stimulation, and the initiation of appropriate responses con- 

 stitute the fundamental work of nervous tissue (Fig. 30, A, D). 

 In some of the lower Metazoa the same cell may do all these tasks. 



84. 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 essential parts 

 merely outgrowths of the ganglion-cells. These outgrowths are 

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

 branched structure; and the axon, or nervous fibre proper. The 

 ganglion cell with its dendrons and axons make up a neuron. It 

 is believed that the whole nervous system even in the higher ani- 

 mals is merely a system of neurons in connection. The impulses 

 can go in only one way : into a cell by way of dendrons and out of 

 it by way of axons. From the axon of one cell they may pass 

 to the dendron of the next; but not in the opposite direction. 

 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 connections. Such are called 

 non-medullated or gray fibres. There are usually however one or 

 more protective sheaths formed about this essential axis: (i) 



Questions on Fig. 30. — 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 (5 and C) ? What is meant by a neuron? 



