CHAPTER II 



THE FROG (continued) 



We have treated the skeleton of the frog in detail, but for the 

 present we will pass the remainder of its anatomy in somewhat 

 rapid survey, leaving details for further study at a later time. 



On making an incision through the muscular sheet which 

 covers the abdomen, we open up a spacious cavity, in which 

 the gut and other organs lie. Extending the cut farther 

 forwards, and cutting through and removing the breast-bone, we 

 find that the cavity is prolonged forward into the region of the 

 heart, and that the heart lies in a part of it which is cut off 

 from the abdominal part by a partition formed by the insertion 

 of some of the abdominal muscles on the oesophagus. This 

 cavity is the pleuro-peritoneal cavity, or coelom. It must be 

 noticed that it does not contain blood, and that it is lined by 

 a smooth, glistening, pigmented membrane — the peritoneum. 

 Tracing the peritoneum round to the dorsal side, we find that 

 under the vertebral column, in the mid-dorsal line, it is folded 

 ventral wards on each side, and the two folds are so closely 

 applied that they form an apparently single sheet of trans- 

 parent tissue, the mesentery, by which the gut is suspended in 

 the coelom. The other organs which lie in the ccelom — ovaries, 

 liver, etc. — are similarly suspended from the dorsal wall by a 

 reflection of the peritoneal membrane, which passes right 

 round them. It should be clearly understood that the gut 

 and other organs which appear to lie in the ccelom are in 

 reality external to it, being separated from it by the mesenterial 

 fold. This will be clear if we liken the abdomen of the frog 

 to a stout bag with an inner lining representing the peritoneum. 

 The gut, etc., do not lie in the cavity, enclosed by the lining, 

 but are placed between the lining and the outer wall of the 

 bag. The gut lies in a deep fold of the lining which projects 

 into the cavity, and the upper or suspensory limbs of the fold 

 are closely pressed together, and form the mesentery. These 



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