BODY TISSUES 301 



that acid-forming bacteria flourish on carbohydrate residues, and 

 that tooth structures are impaired by the action of acid. 



Epithelial and Connective Tissues 



These tissues serve to bind and hold the softer body tissues togetlier. 

 They are characterized by great strength, toughness, and elasticity. 

 Epithelial tissues serve as a covering and lining for the body; for 

 example, these tissues line the respiratory tract and alimentary canal. 

 The outer covering of the body also consists of epithelial tissue and 

 includes such epidermal tissues as skin, nails, hoofs, horn, hair, and 

 feathers. All these tissues are composed of a tough, insoluble albu- 

 minoid protein, called keratin. The keratins of skin are known as 

 pseudokeratins, whereas those in hair and nails are called eukeratins. 



The most important connective tissues are ligaments, tendons, and 

 cartilages. Tendons and ligaments consist of two types of tissue, 

 namely, white fibrous tissue and yellow elastic tissue. The predominat- 

 ing components of tendons and ligaments are the albuminoid proteins, 

 collagen and elastin. Collagen is a tough, insoluble protein which 

 differs from keratin in that it can be hydrolyzed by digestive enzymes. 

 It contains less sulfur than keratin and forms gelatin when treated 

 with hot water. Commercial gelatin is manufactured from collagen- 

 containing animal tissues. 



Tendons contain relatively large amounts of collagen, an albumi- 

 noid which contributes strength. Another albuminoid in tendons is 

 tendomucoid. Cartilaginous tissues contain elastin, chondromucoid, 

 and chondroalbuminoid. Chondromucoid is a glycoprotein which 

 yields, on hydrolysis, glucuronic acid, galactosamine, acetic acid, and 

 sulfuric acid. Chondroalbuminoid has physical properties similar to 

 those of keratin and elastin, but ii can be hydrolyzed by proteolytic 

 digestive enzymes. 



MUSCLE TISSUE 



This important tissue constitutes about 45 per cent of the body 

 weight in man. From 50 to 75 per cent of the total body metabolism 

 occurs in this tissue. The highest metabolic rate accompanies strenu- 

 ous physical exercise, whereas metabolic activity is lowest during 

 complete rest. 



Muscle tissues can be classified into three types, depending on their 

 physiological characteristics. Although they differ to some extent in 



