THE DIGESTIVE AND RESPIRATORY SYSTEM 265 



study of the various steps in the process. In a simple, straight 

 intestine, the starting point of all forms, the tube is enwrapped 

 by the peritoneum (serosa), which becomes reflected along 

 the mid-dorsal line, the two layers becoming applied to one 

 another to form a suspensory ligament, the mesentery, which 

 in turn is attached along the medial line of the body wall 

 ventral to the vertebral column. 



As the tube elongates it lengthens the free edge of this mes- 

 entery, the effect of which is to throw it into sinuous curves 

 directed alternately to right and left, which in extreme cases 

 fall from side to side in the form of long and narrow loops. 

 The intestinal windings are never seen in this typical form, 

 however, owing to the peristaltic movements of the musculosa 

 which cause the folds and loops to constantly change their po- 

 sition so that their disposition in an animal is never the same 

 at two intervals of time.* 



The subdivisions into which the canal is divided anatomi- 

 cally for descriptive purposes depend upon localized enlarge- 

 ment or constrictions, the formation of diverticula, or the 

 presence of definite digestive glands, and become more definite 

 and numerous in higher forms as these features gradually 

 appear and become more emphasized. The first portions to 

 differentiate are the pharynx and stomach, the former being 

 a funnel-shaped enlargement of the anterior end, characterized 

 by pairs of lateral diverticula, the pharyngeal pouches, which 

 may break through to the exterior, and form slits ; the latter 

 a spindle-shaped or sac-shaped compartment for the reception 

 of food of all sorts in about the condition in which it is swal- 

 lowed. The narrowed portion between these forms the oesopha- 

 gus. At its lower end the stomach is bounded by a restricted 

 portion, the pylorus, which, by a specialization of the circular 



* This constant change of appearance and endless variety in arrangement 

 is exactly suited to the demands of divination, which always depends 

 upon a large amount of chance variation of some object; and it is very 

 probable that the Roman augurs, who manufactured prophecies from the 

 inspection of the entrails of the sacrificial animals, were possessed of 

 as definite a system as is seen to-day in the case of palmistry, a " science " 

 founded like the other upon the erroneous and utterly baseless assumption 

 of the causal relation of two unrelated sets of phenomena. 



