Biology of Man 465 



loss of water, disease-producing organisms (bacteria, molds, parasites, 

 etc.), (4) prevention of the absorption of various deleterious materials 

 from our environment, (5) aid in normal respiration, (6) supply infor- 

 mation about our environment through the various types of sensory 

 end organs, and (7) to produce hair, nails, glands, and teeth each with 

 their specalized functions. 



The skeletal system consists of 206 named bones, cartilage, and liga- 

 ments, the latter to hold the other parts together and bind them into an 

 efficient structure. The bones are illustrated in Fig. 229 and the names 

 and numbers are given in table form so as to be easily memorized. It 

 will be observed that the bones may be classified as to shape as follows: 

 (1) long (arms, legs), (2) short (wrist), (3) flat (shoulder blade, 

 patella, etc.), and (4) irregular (vertebrae). It will be noted that the 

 teeth are not listed as part of the skeleton but are included with the 

 integument because of their epithelial origin. By studying a human 

 skeleton it will be evident that there are several types of joints, each 

 with its specific functions. Joints may be classed as ( 1 ) immovable 

 (irregular, dovetail connections [sutures] of the bones of the cranium) 

 and (2) movable, with movements of various types for specific purposes. 

 Movable joints may be further classed as (1) ball and socket (femur 

 and pelvic girdle, humerus and pectoral girdle), (2) hinge (femur arid 

 tibia, humerus and ulna), (3) sliding (most of vertebrae), and (4) 

 rotating (radius and ulna). 



Bones, for the most part, originate in the embryo as cartilage; hence 

 they are known as cartilage bones in contrast to the less common mem- 

 brane bones which are formed by the gradual ossification of soft, fibrous, 

 membranous tissues (skull bones). Certain parts of the skeleton remain 

 as cartilage, such as the external ear, tip of the nose, tip of the breast 

 bone, between the vertebrae, and articulatory surfaces of movable joints. 

 The great strength, elasticity, and reduced friction make cartilage an 

 efficient part of the skeleton. 



Briefly stated, the functions of the human skeleton are (1) to form a 

 framework to support other organs and give posture to the body, (2) 

 to give protection to vital organs (such as brain, spinal cord, heart and 

 lungs), (3) to form solid attachments for muscles so that they may act 

 as a system of levers in motion and locomotion, (4) to store fat in the 

 "fat" marrow, (5) to store certain mineral reserves, (6) to form blood 

 corpuscles by the bone marrow, and (7) to transmit sound waves as 

 accomplished by the hammer, anvil, and stirrup bones of the ears (Fig. 

 250). 



