38 



ZOOLOGY 



SECT. 



endowed in the living state with the property of contractility, by 

 virtue of which, when stimulated in certain ways, it contracts in 

 the direction of its length, becoming shortened, and, at the same 

 time, thickened (Fig. 25). The extremities of the muscle are 

 frequently composed, not of contractile muscular fibres, but of a 

 form of strong fibrous connective tissue the tendon of the muscle. 

 The ends of the muscle are usually firmly attached to two different 

 parts of the jointed framework or skeleton, external or internal ; 

 and, when the muscle contracts and becomes shortened, these two 

 parts are drawn nearer to one another. 



In all but the most lowly-organised animals there is a system 

 of organs the nervous system by means of which a communi- 

 cation is effected between the various parts of the body, enabling 

 them to work in harmony, and by means of which also a communi- 

 cation is established 

 between the organism 

 and the external 

 world. The two 

 essential elements of 

 the nervous system 

 the nerve-cells and 

 nerve-fibres have a 

 regular arrangement 

 which varies in the 

 different animal types 

 both as regards 

 structural details and 

 the relations borne to 

 the other systems of 

 organs ; but there are 

 to be recognised two 

 chief parts or sets of parts the central and the peripheral. 



The central parts of the nervous system consist (Fig. 2G) of 

 certain aggregations of nerve-matter known as nerve-ganglia, 

 containing a large number of nerve-cells ; a relatively large mass 

 of this matter may be collected together to form a brain. To or 

 from these central parts pass all the systems of nerve-fibres,, con- 

 stituting the peripheral part of the system ; the central parts have 

 the office both of receiving impressions conveyed by the nerve-fibres 

 from the surface, from the organs of special sense, and from the 

 internal organs, and of sending off messages through similar channels 

 to the various parts of the body to muscles, to glands, to alimentary 

 canal, and to vascular system. When a movement is to be effected 

 a message passes from the nerve-centre along a nerve-fibre to a 

 muscle and causes it to contract ; when an organ requires the 

 amount of blood supplied to it to be increased or diminished a 

 message is conveyed along a nerve-fibre and causes the dilatation 



FIG. 25. Bones of the human "arm and fore-arm with the 

 biceps muscle, showing the shortening and thickening of 

 the muscle during contraction and the consequent change 

 in the relative position of the bones viz., flexion of the 

 fore-arm on the upper arm. (From Huxley's Physiology.) 



