70 



MORPHOLOGY OF THE VERTEBRATE BODY 



D- 



FiG. il. — Diagram 

 showing action of 

 antagonistic mus- 

 cles which keep the 

 human body erect. 



Arrows indicate the 

 direction of the pull, the 

 feet serving as a fixed 

 basis of support. The 

 muscles .4, B, H, and 

 C keep the body from 

 falling forward; D, E, 

 F, and G keep it from 

 falling backward. 



(After Huxley, re- 

 drawn from Hough 

 and Sedgwick, "Human Mechanism 

 mission.) 



although the exoskeleton reverses the manner of 

 attachment of the musculature. In some animals, 

 like the earthworm, there is no skeleton what- 

 soever. 



The vertebrates thus possess the external 

 features which have been described and which 

 are seen in many familiar animals. They 

 differ externally from animals like insects and 

 other invertebrates in such features as the 

 skeleton, appendages, and sense-organs. In- 

 ternally, the vertebrate plan-of-body has its 

 own peculiarities, notably the type of skele- 

 ton, and the circulatory^ respirator}", and urino- 

 genital systems. It also exhibits features like 

 the dorsal, tubular, central nervous system, 

 which are characteristic of the Chord at a. The 

 schematic representation of a vertebrate (Fig. 

 29) illustrates this structural organization that 

 all vertebrates possess in common. In study- 

 ing the frog or any backboned animal, one 

 may observe this fundamental organization. 

 Since man is a vertebrate, the essentials of 

 human anatomy may be illustrated by the 

 stud}^ of another vertebrate animal like the 

 frog. Since structure has meaning onl}^ in the 

 Ught of function, we have, b}^ the various 

 references to functional activities in the pres- 

 ent chapter, secured an introduction to the 

 problems of physiology which are discussed in 

 the chapters that follow. 



copyright, 1918, by Ginn and Co., printed by per- 



