24 THE INDIVIDUAL ORGANISM 



function is to produce motion. This requires not only a mass of voluntary 

 muscle tissue but also sustentative tissues to hold the muscle tissues 

 in proper arrangement, to protect them from friction, and to connect the 

 bundles of muscle cells with the skeleton. There must be blood and blood 

 vessels to supply fuel and materials for maintenance, and to remove 

 wastes. Nervous tissues are also required to coordinate and control the 

 activities of these tissues and that of the muscle as a whole. 



A single muscle is rarely capable of performing any useful action by 

 itself. Even the simplest voluntary movements involve sets of muscles, 

 skeletal supports, and joints. The mechanical movements of the body are 

 produced by hundreds of different muscles, attached to definite points on 

 a bony framework comprising fixed and movable parts. Together the 

 muscles and bony framework make up a motor or locomotor system, 

 composed of many precisely coordinated organs. 



Other examples of organs are the eyes, ears, organs of smell and taste, 

 and many other "sense organs" that, together with the brain, the spinal 

 cord, and other parts, make up the nervous system. Again, the mouth, 

 esophagus, stomach, liver, pancreas, intestines, etc., are organs that are 

 united and coordinated into a digestive system. 



The integrating physiological systems of the human body. Just how 

 many and what systems are to be recognized in the human body is to 

 some extent a matter of how we choose to subdivide and classify. The 

 number is perhaps determined in part by whether we are more influenced 

 by structural or by functional considerations. From 8 to 10 are commonly 

 distinguished, and we shall compromise on nine. These are: 



1. The locomotor system, which supplies protection and support for 

 other body parts and provides for motion, locomotion, and heat 

 production. Often listed separately as the skeletal and the muscular 

 systems. 



2. The skin or integumentary system, adapted to perform a number of 

 rather diverse functions that are related and correlated by the 

 necessity of their location at the body surface: 



a. Protection of the body against water loss, light, mechanical 

 injury, and the attack of parasitic organisms. 



b. Heat regulation — a function performed in cooperation with the 

 circulatory and nervous systems — a thermostatically controlled 

 radiator. 



c. Sensory perception — the skin is the location and support of a 

 multitude of varied sense receptors for touch, temperature, etc. 



3. The digestive system, which has for its function the ingestion of 

 foods, their processing into absorbable component parts, and the 

 delivery of these end products to the circulatory system. 



