THE RESPIRATORY SYSTEM. 



257 



laryngeal sacs are normally developed in anthropoids after birth, 

 and attain in them great dimensions. 



The Diaphragm. — -The diaphragm is developed from five 

 elements — one mesial from the primitive mesentery, two ventro- 

 lateral parts formed by adhesion, and two dorso-lateral formed 

 much later (Fig. 210). In fishes, only the mesial or mesenteric 

 element is developed ; in reptiles and birds the ventro-lateral 

 elements are added, these three parts forming the septum 

 transversum. In mammals the dorso-lateral elements are de- 

 veloped, and thus the pleural cavities become separated com- 

 pletely from the peritoneal. Occasionally in man the dorso- 

 lateral elements may fail ; a communication remains between the 

 pleura and peritoneum, through which the abdominal contents 

 may become herniated into the pleural cavity. This occurs eight 

 times more frequently on the left than on the right side. The 

 explanation will be found in the manner in which the liver is 

 developed. 



Elements entering into the Formation of the Diaphragm. 

 1. The Mesial or Mesenteric Element (Pig. 210). — When the 

 splanchnopleures meet below so as to enclose the fore-gut, they 



vitelline 

 uem 



dorsal mesentery 

 coelom 

 stomach 

 vitelline vein 

 somatopleure 



liver bud 



uentral mesent. 

 (splanchnopl.) 



Fig 211 —Diagrammatic section behind the Embryonic Heart to show the Part of the 

 primitive Mesentery which forms the mesial Element of the Diaphragm. 



form a mesial septum between the right and left halves of the 

 coelom, with the gut suspended in it (Figs. 210 and 211). The 

 part between the gut and the spine is the dorsal mesentery ; the 

 part of the septum between the gut and the ventral wall is the 



R 



