444 BIOLOGY. 



2682. Soaring is creeping in the air. 



2683. Hovering is standing in the air. 



2684. Pouncing is hopping in the air. 



2685. Diving is hopping in the water. 



2. Functions of the Muscular System. 



2686. The muscular system performs, in an active 

 sense, what the osseous system does in a passive. The 

 strength or power of the muscles, and their leverage 

 attachment, is here especially to be regarded. The con- 

 traction of the fibres is a charging, by nerves and blood, 

 of the two fibre-poles. 



2687. The fibres are charged by the air. It 

 is, in the most general sense, the respiratory vessels by 

 which the muscle is charged. This is strictly the case in 

 Insects, where the tracheae traverse all the limbs, and 

 directly conduct to the flesh the polarity of the air. In 

 animals, however, with a closed circulation, the arteries 

 undertake the conveyance of air upon the blood, and it 

 is then the latter fluid which streams into the muscles in 

 order to charge them. 



2688. Thus if an artery be ligatured, the limb is 

 crippled or lamed. The artery, however, imparts only 

 the positive pole, and consequently of itself produces no 

 shortening or contraction of the fibres. The oxydation 

 takes place at the lower end of the muscle ; here, there- 

 fore, the latter passes over into tendon. 



2689. The nerve is the second condition of the muscular 

 contraction, since it evokes in the fibre the negative 

 pole. Thus if a nerve be ligatured, the limb is likewise 

 motionless. 



2690. If the poles be brought by contraction into close 

 approximation, the fibres must re-extend, so soon as the 

 influence of the blood or the nerve ceases. 



2691. Now since the blood is constantly streaming in, 

 the reason for the muscular rest must reside in the 

 nerves. The rationale of voluntary motion is conse- 

 quently the nerve. The relaxation or extension is an 

 unloading of the fibres. 



