THE STRUCTURAL AND FUNCTIONAL SYSTEMS 



69 



tate joints that permit motion in more than one plane and a com- 

 plex of muscles working together in a coordinated fashion. When 

 one understands the principles involved, many interesting demon- 

 strations may be obtained merely by moving various parts of the 

 body and observing the tightening and slackening as the muscles 



^V. C, 



T. 9 



Fig. 40. — A. Axial skeleton of man in lateral view, with skull and spinal 

 column cut in n;edian section. B. Skeleton of frog from dorsal view. 



as, astragalus; c.b., cartilage bone; ca, calcaneus; cr, cranium; /, femur; h, humerus; 

 i, ilium; m.b., membrane bone; pi, pelvic girdle; r-a, radio-ulna; s, scapula; sa, sacrum; 

 /-/, tibio-fibula; th, thoracic region; u, urostyle; v.c, vertebral column; v.l, 1st vertebra 

 or atlas; d.9, 9th or sacral vertebra. (A, redrawn from Hough and Sedgwick, "Human 

 Mechanism," copyright, 1918, by Ginn and Co., printed by permission. B, redrawn from 

 Parker and Haswell, "Te.xtbook of Zoology," copyright, 1921, by Macmillau and Co., Ltd., 

 printed by permission.) 



contract and relax beneath the skin. Although the mechanical 

 arrangements are many and varied, the muscular movements 

 recognizable in the bodies of animals are all dependent upon the 

 fundamental phenomenon observed in the shortening and thicken- 

 ing of any single muscle and its effect upon the skeleton and sur- 

 rounding parts. The same principle appears in the movements 

 of an animal like an insect, with its external skeleton (Fig. 22), 



