72 



THE ORGANIZED ANIMAL 



Sustentative tissues. These are the tissues 

 that give the body form and support. They 

 are composed of cells imbedded in a matrix, 

 secreted by the cells and usually occupying 

 more space than the cells themselves. The 

 matrix may be composed of fluid, gelatinous 

 material, long tough fibers, or hard miner- 

 alized material. There are several different 

 kinds of sustentative tissue, all dependent 

 on the type of matrix ( Fig. 4-4 ) . 



The tough ligaments that fasten the 

 bones tosether and the tendons that con- 

 nect the muscles to the bones are composed 

 mostly of tough fibers forming a matrix 

 about the cells which produce them. Also, 

 many of the internal organs of the body 

 are laced together by sheets of similar tissue 

 called mesenteries. In this type of tissue the 

 fibers lie at random with no particular 

 arrangement, which results in a thin layer 

 of tissue that is soft and pliable, yet tough. 

 A similar type makes up most of the deeper 

 portions of the skin lying below the super- 

 ficial epithelium. It is this sustentative 

 tissue in its protective covering that gives 

 the skin the qualities essential for an ade- 

 quate body covering. 



Animals, particularly land forms, require 

 a very rigid skeleton to support their mas- 

 sive weights. This is provided by bone and 

 cartilage, types of sustentative tissue that 

 are composed of large quantities of matrix 

 secreted by isolated cells. In the case of 

 cartilage the matrix is a spongy semi-solid 

 mass in which cells are embedded in tiny 

 cavities (lacunae). The cells are usually 

 single, although as they divide there may be 

 as many as four in one cavity before they 

 finally separate. Cartilage is excellent ma- 

 terial to resist shock; therefore, it is found 

 between bones such as the vertebrae. It also 

 provides ideal support for the tip of the 

 nose and the external ear where retention of 

 shape and pliability are essential. Bone, on 

 the other hand, consists of a mineralized 

 matrix (calcium carbonate and phosphate) 

 which is very rigid, imparting an element of 

 solidarity to the entire structure. In this 



case also, the matrix is formed from cells 

 embedded in tiny spaces (lacunae) within 

 the matrix itself. These usually take on defi- 

 nite patterns around blood vessels and 

 nerves, called Haversian systems. All of 

 the cells have access to a food supply from 

 the blood system by means of tiny canals 

 (canaliculi), for these cells are alive and 

 must be nourished like any other cells 

 (Fig. 4-4). 



Tissue in which fat is stored is often 

 classified as sustentative tissue, primarily 

 because there seems to be no other category 

 for it. It performs no mechanical function 

 other than to occupy space. The fat is 

 stored within the cell itself and these cells 

 are located under the skin and in the ab- 

 dominal region as well as other well-known 

 areas of the human body. During periods 

 of starvation it is very scanty, but during 

 good times it may be stored in quantities 

 far beyond any usefulness to its owner, as 

 attested by many overweight people. 



Contractile tissue. This tissue, called 

 muscle tissue, has the ability to shorten, 

 that is, to pull its two ends closer together. 

 This apparently very simple action is re- 

 sponsible for all of the movements of most 

 organisms. Mviscle tissue consists of elon- 

 gated cells or fibers whose internal parts 

 consist of myofibrillae, tiny contractile 

 fibrils, lying in a fluid protoplasm called 

 sarcoplasm. There are three well-defined 

 kinds of muscle tissue, visceral, skeletal, 

 and cardiac, each of which differs in its 

 appearance under the microscope (Fig. 

 4-5). 



Visceral muscle is found in the walls of 

 the digestive tract, and other places in the 

 body which are not under voluntary nerv- 

 ous control; this activity is primarily auto- 

 matic, and is not under the influence of the 

 will. The cells are spindle-shaped with cen- 

 trally located flattened nuclei, and with 

 myofibrillae running lengthwise in them. It 

 is the shortening of the myofibrillae that 

 pulls the two ends of the muscle cell closer 

 toeether. Visceral muscle contracts and re- 



