276 



BIOLOGY AND HUMAN LIFE 



Trachea 



Lungs 



Liver 

 {turned 

 back) 



Gall 

 bladder 



trunk are the viscera, or the organs directly connected with the 

 main functions of nutrition (including respiration, circulation, 

 and excretion) and reproduction (Figs. 125 and 126). 



In fishes, which have no lungs but breathe through the gills (see 

 Fig. 186), the viscera of the digestive system lie fairly well forward 

 (see Fig. 78). The dissolved oxygen, which is taken from the air by the 



water, is absorbed 

 from the water by 

 osmosis. Among all 

 vertebrates there is, 

 of course, a portion 

 of the food tube con- 

 necting the mouth 

 and the stomach 

 {esophagus) that 

 passes through the 

 thorax. Among ver- 

 tebrates that have 

 no diaphragm (all 

 classes of backboned 

 animals except mam- 

 mals) there is no 

 sharp distinction be- 

 tween the thorax and 

 the abdomen, and 

 we find a great deal 

 of overlapping of 

 organs. 



Running through 

 both thoracic and 

 abdominal cavities 

 is a double chain of 

 nerve ganglia mak- 

 ing up the ''sympathetic nervous system" (see page 253), which lies at 

 the very back of the cavities, in front of the spinal column. 



In all except the lowest vertebrates the head carries a dis- 

 tinct jaw and several special sense organs, the eyes and the 

 ears being the most prominent. The larger part of the head 

 in man consists of the brain box, or cranium. In the frog you 



Heart 



I 



Diaphragm 



Stomach 



Pancreas 



Large 

 intestine 



Small 

 mtesline 



Appendix 

 vermiformis 



Fig. 126. The viscera of man 



In all air-breathing vertebrates the internal organs have 

 essentially the same general structure and the same gen- 

 eral arrangement 



