384 



ORGAN SYSTEMS OF MAN 



inside. Thus the bone gradually becomes 

 longer and increases in diameter. 



Although bone may seem dead, it is far 

 from it, as was pointed out earlier (p. 72). 

 The Haversian system (Fig. 4-4) consists 

 of a canal in the center containing blood 

 vessels and a nerve, surrounded by concen- 

 tric rings of bony matrix, and between them 

 scattered tiny spaces, lacunae, filled with 

 the bone cells. Very tiny tubes (canaliculi) 

 connect the bone cells with one another and 

 the central canal, and it is through these 

 canals that the cells are nourished and kept 

 alive. These bone cells secrete the bony 

 matrix in which they are entombed. It is as 

 if a mason were to surround himself with 

 a concrete wall of his own building and 

 thus be enclosed in a chamber which he 

 could never leave, but in which he would 

 be kept alive by small portals through 

 which nourishment could be supplied. 



Ability to move 



Nearly all animals from amoeba to man 

 have the ability to locomote, and the few 

 which lack this are still able to move some 

 parts of their body (Fig. 15-6). Amoeba 

 moves by a complex sol-gel reversal mech- 

 anism which causes the pseudopodia to ex- 

 tend and retract. In addition to being able 

 to move its body in a worm-hke manner, 

 Euglena has a contractile flagellum which 

 propels it through the water. Paramecium is 

 provided with numerous cilia that beat in 

 unison to bring about its erratic move- 

 ments. Hydra is the first animal with cells 

 that contain muscle fibers which contract 

 along an axis. It is the combined action of 

 the many neuromuscular cells that makes it 

 possible for this animal to contract and to 

 extend itself in its movements. Once this 

 type of movement, that is, muscular contrac- 

 tion, had appeared in animals, it persisted 

 through all subsequent forms. We shall, 

 therefore, spend some time in studying mus- 

 cles and their operation, and again man will 

 serve our purpose as well as any other 

 animal. 



THE MUSCULAR SYSTEM 



One of the most striking characteristics 

 of animals is movement. Since they are 

 voracious feeders they must be on the move 

 most of the time in search of food, and, 

 movement is thus imperative to their con- 

 tinued existence. Among all but the Proto- 

 zoa and perhaps a few others, contracting 

 muscles are responsible for movement, not 

 only of the body as a whole and its external 

 appendages, but the internal organs as 

 well, such as the organs of digestion and 

 circulation. It is not surprising, therefore, 

 that a man's body has more than 600 sepa- 

 rate muscles. 



The way muscles work 



The muscle responds like a rubber band; 

 it can do only one positive thing and that is 

 contract. When it is not contracted it is said 

 to be relaxed. The function of a muscle, 

 then, is to pull two objects closer together. 

 This means that there must be muscles 

 which pull bones in one direction and those 

 which pull the same bones back again ( Fig. 

 15-7). Muscles working against one another 

 are said to be antagonists. For example, by 

 contraction of the large muscle in the front 

 of the upper leg the bent leg straightens, as 

 in kicking a ball. Once the leg is straight it 

 must be bent again before another step or 

 kick can be executed, and several large 

 muscles on the back side of the leg carry 

 out this movement. To be sure, there is no 

 complete relaxation of one set of muscles 

 during the contraction of their antagonists. 

 Both contract some, the resultant action 

 depending on how much each contracts. 

 When bones are bent on one another the 

 action is spoken of as flexion; when they 

 are straightened out the action is described 

 as extension. The example of kicking is a 

 case of extension and flexion of the leg 

 bones. Likewise, the closing of the hand is 

 flexion; the reverse or opening of the hand 

 is extension. Although there are many other 

 types of muscle action, antagonistic ac- 



