THE GASTRO-PULMONARY 



SYSTEM. 



GENERAL CONSIDERATIONS. 



The food-stuffs required to compensate the continual loss occasioned by the 

 tissue-changes within the body are temporarily stored within the digestive tube. 

 Durmg this sojourn the food is subjected to the digestive processes whereby the sub- 

 stances suitable for the nutritive needs of the animal are separated by absorption from 

 the superfluous materials which, sooner or later, are cast out as excreta. Closely 

 associated with digestion, and in a sense complementary to it, is the respiratory func- 

 tion by which the supply of oxygen is assured. In the lowest x'ertebrates these two 

 hfe-needs, food and oxygen, are obtained from the water in which the animal lives, 

 this medium containing both nutritive materials and the air required for the perform- 

 ance of the respiratory interchange of gases (oxygen and carbon dioxide). 



Fig. 1281. 



Wolffian body 



Spleen 



Notochord 



Oral cavity Pharyngeal / , 



pouches Heart ^""S| 



Cloacal orifice 



Liver 

 Stomach Pancreas 



Mid-gut „■ J 



'' Hind-gut 



Sagittal section of schematic vertebrate {Modified from Fleischmann.) 



Since, therefore, in these animals both food and oxygen are secured from the 

 same source, the water, the diges,ti\^e and respiratory organs form parts of a single 

 gastro-pulmonary apparatus. This close relation is seen in the lower vertebrates 

 (fishes), in which the anterior segment of the digestive tube is connected on either 

 side with a series of pouches and apertures, the branchial clefts, bordered by the 

 vascular gill-fringes by means of which the blood-stream is brought into intimate 

 relation with the air-containing water. 



When the latter element is forsaken as a permanent habitat and the animal 

 becomes terrestrial, a more highly specialized apparatus, suited for aerial respiration, 

 becomes necessary. This need results in the development of the lungs. The latter, 

 however, retain the intimate primary relation to the digestive tract, and are formed 

 as direct ventral outgrowths from the gut-tube. 



The vertebrate digestive tract early becomes differentiated into three divisions : 

 fore-gut, vtid-g7it, and hind-gut. The first includes the mouth, pharynx, oesopha- 

 gus, and stomach, and serves for the mechanical and chemical preparation of the food 

 materials. The second comprises the longer or shorter, more or less convoluted 

 small intestine, and forms the segment in which absorption of the nutritive materials 

 chiefly takes place. The third embraces the large intestine, and contains the super- 

 fluous remains of the ingested materials which are discarded from the body at the 



1527 



