10 



ZOOLOGY. 



or by one end to an integumentary structure (hair, 

 feather, scale), and to parts of the skeleton. The 

 animals which are devoid of any internal skeleton, 

 the inveHebrates (i.e. all animals except vertebrates), 

 naturally possess no skeletal muscles. Examples of 

 derTnal muscles are those by means of which a bird 

 erects its feathers (tail-coverts of peacock!), and 

 those which enable a hedgehog to roll itself into a 

 ball and stick out its spines. Each end of a skeletal 

 onuscle is connected with a bone. If such a muscle 

 contracts the more easUy movable bone is drawn 

 towards the less easily movable one (Fig. 4). In 

 order that the bones may be movable upon one 

 another they are united together by joints. 



According as muscular movements are, or are not, 

 under the influence of the will, they are distinguished 

 as volvmtary and involuntary. To the latter kind 

 belong the movement of the heart, and the movements 

 of the muscles in the wall of the gut by which the 

 food is made to progress. 



To destroy the contractile power of a muscle it is 



not necessary to injure 

 the muscle itself. Every 

 muscle is related to a 

 nerve, which sends its 

 fine branches to the 

 fibres making up the 

 muscle. If we cut the 

 nerve, the correspond- 

 ing muscle loses its 

 power of contraction. 

 But the nerve arises 

 from the central nervous 

 system, which in ver- 



; .t^S™'"",? "' .""^ -*™ ■'y Contraction tohva + PS nri n pi r,o 1 1 tt ,.«« 

 of the Biceps Mnaolc. a, humerus ; 6, ulna ; "° orates principally COn- 



c, elbow-joint; a, Keeps muscle ;«, origin;' sists of the brain atirl 



/.insertion of the same. In the right-hand . , "iaiu anu 



figure of the muscle (J is contracted; in the Spinal COrd. The 



left-hand figure it is slackened. ^^^^^^ ^-^ therefore 



lose its contractile power if the connection with 



