Chap. 10 MOVEMENT MUSCLES 165 



dramatically important. Those of the uterus are responsible for birth. They 

 hold blood in the vessels at a regulated capacity, thus largely maintaining blood 

 pressure and the circulation of blood. Attacks of asthma are spasmodic con- 

 tractions of smooth muscles that under normal nervous control regulate the 

 amount of air in the bronchioles. Less serious but vivid in experience are the 

 contractions in the walls of the stomach that cause hunger pains. 



Muscles of Some Familiar Invertebrates 



Smooth muscles are located in the viscera and the body wall of many inver- 

 tebrates. Clams, mussels, and oysters can hold their shells closed for long 

 periods, some of them for days at a time. The shells of all bivalves are hinged, 

 and in the hinge is an elastic band which continually resists the closing of the 

 shells. This resistance is met by the tonic contraction of adductor muscles at- 

 tached at either end to the inner surface of the shells. The large adductor mus- 

 cles of the scallop (Pecten) are familiar as fried scallops. Experimental stimu- 

 lation of these muscles indicates that they contain certain rapidly contracting 

 muscle cells along with a majority of slowly contracting ones. This combina- 

 tion is ideal for the lively habits of scallops which, by clapping their shells 

 together and rapidly expelling the water between them, are able to skip out 

 for short distances through the water by a kind of jet propulsion. Involuntary 

 muscles with a very different function take part in the "blushing" of the squid. 

 When these handsome relatives of the devilfishes are excited, glimmering 

 flashes of pink and red shift over their bodies due to the movements of pig- 

 ment (in chromatophores) controlled by muscles. 



The movements of the common earthworm are an easily observed example 

 of peristalsis, i.e., successive waves of contraction of the rings of smooth mus- 

 cle in the body wall. Close to these, layers of longitudinal muscles extend the 

 length of the worm. When the long ones contract, the fluid-filled body of the 

 worm shortens and bulges; when the circular muscles contract, they squeeze 

 the body to slenderness and drive the fluids forward and backward forcing it 

 to elongate. 



Insects have the most complex muscular systems and most clearly striated 

 muscle of all invertebrates. The number of distinct muscles is very large, vary- 

 ing in different insects, but there are often over 2,000. In a dissection, muscle 

 is one of the most conspicuous tissues of the insect body. It is either colorless 

 and transparent, or yellowish white, often soft, almost gelatinous, notwith- 

 standing its efficiency. 



Patterns of Vertebrate Locomotion 



No other animals take such long journeys by sea and land as the vertebrates; 

 eels swimming down streams and half across the Atlantic; birds flying from 

 Alaska to the Argentine; and human populations moving to distant lands. All 



