THE STRUCTURAL AND FUNCTIONAL SYSTEMS 51 



tion as well as structure, some account of the manner of action will 

 be included. 



The Digestive System. — The features of the digestive system 

 that are important for the present purpose may be indicated by a 

 brief elaboration of what has been said under the head of CcBlome 

 and Viscera (p. 47). The digestive " tract " is a tube leading 

 from mouth to anus and extending through the coelome, within 

 which it is coiled about and differentiated into the esophagus, 

 stomach, small intestine, large intestine, and cloaca. In a schematic 

 representation, as shown by Fig. 23, the tract may be drawn run- 

 ning straight through the ccelome or with a minimum of folding, as 

 it actually does in the simplest vertebrates {Cyclostomata; Fig. 

 1 E, p. 10) and in the embryonic stages of more complex forms like 

 the frog antl man. Attached to the tract by ducts, and originat- 

 ing as outgrowths during its development (r/. Fig. 217, p. 413), 

 are the liver and the pancreas, which are digestive glands. 

 The connections which these organs have with the digestive cavity, 

 by means of their ducts, are sur\avals of their origin as outgrowths 

 from the gut cavity of the embryo. Their linings are modified 

 portions of the embryonic digestive canal. Externally, they are cov- 

 ered with peritoneum like all the other organs within the coelome. 



The mesenteries of the digestive tract have been described in 

 connection with the peritoneum. They perform the two-fold 

 function of holding the parts in position and serving as a bridge 

 by which nerves and blood vessels pass between the digestive 

 canal and the body wall (Fig. 19, p. 40). The finer structure of 

 the intestinal and stomach walls will be considered in connection 

 with the processes of nutrition and the microscopic organization 

 of the body, which will be discussed in later chapters. 



The Urino-genital System. — In vertebrate animals, the kid- 

 neys and their ducts, the ureters, are so intimately related to the 

 reproductive organs that it is customary to speak of these two sets 

 of organs as the urino-genital system. While this is justifiable 

 on anatomical grounds, it is obvious that such a system includes 

 organs of widely different functions. The kidneys might be 

 classed, along with the skin and lungs, as belonging to an excretory 

 system composed of organs located in different parts of the body, 

 and not connected as are the parts of other systems with which we 

 are familiar. The reproductive organs would then be taken as 

 the reproductive system. 



