GENERAL INTERNAL ORGANIZATION 



49 



teries assume greater complexities, through fusions and oblit- 

 erations, as the coihng of the digestive tract becomes more 

 extensive. An example of the disappearance of the greater part 

 of a mesentery is seen in the stomach region of the adult frog. 

 The simple relationship described is that which occurs in the 

 early stages of development and persists to some extent in the 

 adult. 



AVhen the coelomic region of the frog is compared with the cor- 

 responding portion of the human body (Fig. 28), the most obvi- 

 ous difference is the presence in 

 man of the diaphragm separating 

 the region of lungs and heart from 

 that of the abdominal viscera. 

 The diaphragm is pecuhar to 

 mammals among the vertebrates, 

 as the reader may have observed 

 by noting its absence in the domes- 

 tic fowl, the fish, and any reptile 

 that he may have examined. The 

 other anatomical features of the 

 frog's body that appear in this 

 connection have been sufficiently 

 explained. 



The Vertebrate Plan-of-body. — 

 What is found in the body of a 

 frog is illustrative of what exists 

 in other vertebrate animals. It is 

 possible to construct a generahzed 

 diagram that does not represent 



Fig. 28. — Thoracic and abdominal 

 viscera of man from ventral view. 



di, diaphragm; h, heart; in, small 

 intestine; I, liver; Ig, lung; st, stomach. 

 (Redrawn from Hough and Sedgwick, 

 "Human Mechanism," copyright, 1918, 

 any one vertebrate exactly but by Ginn and Co., printed by permission.) 



shows the essential relationships 



in all vertebrates (Fig. 29). As a subdivision of the Phylum 

 Chordata (p. 36), the vertebrates possess, at some stage in devel- 

 opment, a dorsal, tubular, central nervous system ; anotochord; cr 

 primitive axial skeleton, and gill slits. As vertebrates, they possess 

 a skull, a segmented vertebral colmnn, and two pairs of appendages. 

 They are quite unlike such a group as the Arthropoda, to which 

 the insects and crayfish belong (Fig. 117, p. 240), and in which the 

 greater portion of the central nervous system is ventral, the ccelome 

 is rudimentary, the heart dorsal, the skeleton external, the appen- 



