94 



PRINCIPLES OF ANIMAL BIOLOGY 



Then the circular muscles contract, while the longitudinal ones relax. 

 Since the body cavity (coelom) is filled with a fluid and cannot reduce its 

 volume, contraction of the circular muscles forces the body to elongate, 

 thus pushing the front end forward to take a new hold upon the soil. 

 The setae, as previously explained (page 89), are tilted forward or back- 

 Avard by opposing muscles. In vertebrate animals, bones are moved by 

 muscles and tendons placed on opposite sides of the bones at or near the 

 joints. The arrangement at the knee joint in man is shown in Fig. 83. 

 The flexor muscle bends the joint, the extensor straightens it. When 



NSERTION 



ORIGIN 

 Fig. 83. — Diagram of knee joint in man, illustrating opposed muscles. 



one of these muscles contracts, the other must relax if movement is to 

 be produced. If both contract the leg is merely made tense. The area 

 of attachment of the less movable end of the muscle (usually that nearest 

 the body) is called the origin of the muscle, that of the more movable 

 end its insertion. In such boneless movable parts as the eyelids and lips, 

 one set of muscles, operating to pull radially away from the openings 

 which these structures surround, is opposed by circular bands of muscles 

 which close the openings. The stomach and intestine of vertebrate 

 animals possess longitudinal and circular muscles which operate much 

 as do those of the earthworm. Everywhere muscle is opposed by muscle. 



Fig. 84. — Smooth-muscle cells. 



The necessity of this arrangement arises from the fact that, while muscle 

 contracts vigorously, its expansion is entirely passive. It can force move- 

 ment in one direction but can only permit it in the opposite direction. 



Muscle. — Muscles constitute the contractile tissue referred to in the 

 preceding chapter (page 81). They are nearly always plates or bundles 

 of cells, not single cells. Three types of muscle cells in vertebrate 

 animals may be recognized, known respectively as smooth, striated, and 

 cardiac. 



Smooth muscle is composed of cells each of which is provided with a 

 single nucleus. The cytosome contains well-marked longitudinal fibrils. 

 These cells (Fig. 84) have the form of slender spindles with unbranched 

 tips or in certain organs the tips may be branched. They are found in 



