OF THE MOVEMENTS. ' 127 



arachnides, &c., the difference in structure becomes more and 

 more striking. These peculiarities will be adverted to here- 

 after. 



OF THE MOVEMENTS. 



Muscular Contraction. 



246. By sensation, man and animals perceive the ex- 

 ternal world ; by motion, they react upon it. This latter series 

 of functions is dependent on the property of contractility. 



In some very simply organized animals, every part of the 

 body seems susceptible of contracting and elongating itself, 

 as in the hydra (Fig. 4) ; but as we ascend in the scale of 

 animals we find that the property belongs exclusively to the 

 muscular fibre. Collected in masses these form the muscles, 

 which again constitute the flesh of animals. Their colour, 

 which varies from a deep red to whitish, depends on the pre- 

 sence of the blood, and is not inherent in their structure. 



247. Structure of the Muscles. The muscles are com- 

 posed of fasciculi, or bundles of fibres united by cellular tissue, 

 and these again of bundles of fibres more and more delicate, 

 until an elementary fibre, or what may be considered so, is 

 reached, and this can only be seen by means of the most 

 powerful microscope. It seems as if it were composed of a 

 series of discs. After death the muscular fibre is soft, and 

 easily torn ; during life it is firm and elastic. It is composed 

 essentially of fibrin, to which are united albumen, osmazome, 

 and some salts. 



248. By means of certain stimulants, such as the will, 

 muscles contract ; that is, they swell, shorten themselves, and 

 become extremely hard; the phenomena may readily be ob- 

 served by bending the fore-arm upon the arm. 



The mechanism by which this is effected has not yet been 

 discovered. One thing alone is certain, that the two extre- 

 mities of the fibre or muscle approach each other, and thus, 

 of necessity, act on whatever they may be attached to that is 

 moveable, displacing one or both, and of course the body 

 itself. Hence these organs have been called the active organs 

 of motion, in contradistinction to the bones, which are 



249. The muscles are attached to the bones by fibres of 

 great strength, insensible, and of a dead-white colour; they 

 are called tendinous or tendons ; when membraniform in their 

 arrangement they are called aponeuroses. 



