Chap. 7 TISSUES 119 



is chiefly fat. Red marrow occurs mainly at the ends of long bones. It con- 

 tains fewer fat cells and is characterized by the development of red blood 

 cells and granular white ones. Great numbers of these are continually passing 

 into the blood and a comparable number of worn-out cells is withdrawn. 

 This is an instance of the regulated economy of the body which breaks down 

 comparatively seldom. 



Blood and Lymph 



Blood and lymph, its supplemental fluid, are tissues comparable to connec- 

 tive tissue and the skeletal tissues, bone and cartilage, to which they are 

 related. As here described, there are four types of connective tissue in each 

 of which the cells are surrounded by abundant intercellular substance. In 

 ordinary connective tissue the substance is gelatinous; in cartilage, it is tough 

 and jellylike; in bone, hard; in blood and lymph, a liquid in which the cells 

 float freely. 



As far as its origin and related tissues are concerned, the discussion of 

 blood should be included at this point. Instead it is given in Chapter 12, 

 Blood and Circulation, and is thus placed with the vessels that carry it through 

 the body. 



Muscular Tissue 



Muscle cells are so elongated that they are commonly called muscle fibers; 

 thus, the terms muscle cell and muscle fiber are used interchangeably. A mus- 

 cle fiber is living matter; a connective-tissue fiber is not. Muscle fibers, that is, 

 muscle cells, contain fibrils (myofibrils) within their cytoplasm; the shorten- 

 ing of these is the contraction or muscular action. Muscle cells are usually 

 in bundles held together by connective tissue. Muscle has a high degree of 

 contractility. This fundamental character of protoplasm is evident in the 

 movements of an ameba and the action of its contractile vacuole, as well as 

 in the movements of all other animals. Contraction of protoplasm is accom- 

 panied by chemical and physical changes. 



Chemical Composition of Muscle. About three-fourths of muscle is water. 

 Of the remainder about four-fifths is protein; the other one-fifth includes car- 

 bohydrates and fats, nitrogenous substances (urea, creatine), lactic acid, 

 pigments, enzymes, and inorganic salts. The most abundant protein is myosin 

 which makes up most of the contractile myofibrils. The carbohydrate is largely 

 glycogen, the ready-to-use food stored in many tissues. When a muscle has 

 been excited and fatigued its store of glycogen disappears and an equivalent 

 amount of lactic acid takes its place. When the oxygen supply is renewed 

 and after oxidation occurs the lactic acid is reduced and a proportional 

 amount of heat results. Muscles contain a red pigment, muscle hemoglobin 

 or myoglobin, which has an even greater affinity for oxygen than has the 



