120 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



hemoglobin of blood cells. It is abundant in the "red" muscle of birds and 

 mammals and the heart muscle of all vertebrates. 



Types of Muscle. There are two main types: smooth, unstriated, or invol- 

 untary; and striated, skeletal or voluntary. Cardiac (heart) muscle, although 

 striated, is involuntary and contracts rhythmically. 



Smooth Muscle Cells. These spindle-shaped cells occur in sheets held 

 together by connective tissue (Fig. 7.13). They include muscles in blood 

 vessels, in the urinary bladder, in the bronchial tubes of the lungs, in the 

 alimentary canal, and in other structures not under voluntary control. The 

 contraction of the iris of the eye in bright light is due to the contraction of 

 smooth muscle. The contraction of smooth muscle causes goose flesh, the 

 erection of hairs on the arms resulting from fear or cold, and the vivid lift 

 of hairs on a cat's tail. 



Striated or Skeletal Muscle. This is the muscle attached to the skele- 

 ton, the voluntary type that comprises the bulk of muscle in the body. Most 

 of the meat that we eat is voluntary muscle, cut in slices, actually cross- 

 sections, taken at right angles to the length of the muscle cells (Fig. 7.14). 

 Striated muscle differs from the smooth type in the size and shape of its cells. 

 The most conspicuous microscopic structures are the alternating light and 

 dark crossbands of the cells. Striated muscle fibers are regarded as giant 

 multinucleated cells. Some very long ones have about 100 peripheral nuclei. 

 Each muscle cell contains a bundle of contractile fibrillae. In insects probably 

 all muscle is more or less striated. Striations are prominent in the flight 

 muscles of the honeybee when spread thinly on a slide in their fresh condition. 



ii^^^^^;^:;^mimMi:mm 







Fig. 7.13. Smooth muscle. A, fibers (cells) from a frog's bladder; B, cross sec- 

 tion of smooth muscle from the bladder of a kitten; the muscle cells are held 

 together by connective tissue; the section misses the nuclei of many cells; C, 

 branching smooth muscle cells in the aorta of a dog. x 900. (Courtesy, Nonidez 

 and Windle: Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., 

 Inc., 1953.) 



