Chap. 10 MOVEMENT MUSCLES 157 



the world is filled with cackle, bark, speech, and song. These and other char- 

 acteristics of muscles are matters of great social and economic importance. 

 Muscle is meat, almost the sole food of carnivorous animals and also of high 

 value to man and other omnivorous ones. 



Kinds of Muscle 



There are two main types of muscle, distinguished by their activity and 

 appearance under the microscope. Involuntary or smooth muscle, the older 

 one in the history of muscle, is generally distributed in the invertebrates except 

 arthropods, and occurs in the hollow organs of vertebrates such as the stomach, 

 intestines, and arteries. The pupil of the eye becomes smaller when involuntary 

 circular muscles contract and narrow the iris. Hairs stand up when their erector 

 muscles contract from cold and other causes (Fig. 10.2). 



Voluntary, skeletal, or striated muscles are the bulkier ones. Those of the 

 body wall and arms and legs contribute largely to the form of the body (Figs. 

 9.16, 10.3). 



Cardiac or heart muscle, often named as a third type, is intermediate in 

 structure to striated muscle and in activity to smooth muscle. 



Dermis 



B 



Fig. 10.2. An involuntary muscle, the hair muscle. Diagrams of sections of skin 

 showing the follicle or root of a hair with the muscle attached to the skin and the 

 root sheath. A, hair naturally leans at an angle when the muscle is relaxed. The 

 region about the hair is supplied with nerves associated with the muscle. B. under 

 certain conditions, such as cold and nervous shock the nerves stimulate the hair 

 muscles to contract pulling the hairs up straight and the skin into little hillocks, 

 "goose flesh." 



