THE ARTERIAL SYSTEM. 47 



and the longitudinal folding of its mucous membrane. 

 Pass bristles in from the pancreatic and cystic ducts, 

 and observe that each passes obliquely through the in- 

 testinal wall. 



109. Open the coecum and colon along their free 

 borders ; note the absence of an ileo-colic valve, and 

 the muscular and mucous coats of the tube. 



110. The dorsal aorta may now be followed back from 

 its point of formation (102). Passing between the kid- 

 neys, it ends behind them by dividing into the right and 

 the left common iliacs. In dissecting its branches, which 

 is a matter of some difficulty, disturb the renal and re- 

 productive organs as little as possible. With care the 

 following branches will be found : 



a. Several small twigs to the kidneys and genital 

 glands. 



b. A pair of branches which run dorsally, and 

 enter the canal on each side of the vertebral 

 column, which is arched over by the necks of 

 the ribs. 



c. A branch {epigastric artery) on each side, which 

 runs out dorsal to the kidney ; and, reaching the 

 edge of the carapace, runs forward to anasto- 

 mose with the internal mammary (96, g). 



d. A branch on each side, distributed to the pelvic 

 muscles. 



111. Each of the common iliac arteries, the terminal 

 branches of the dorsal aorta, soon divides into an inter- 

 nal and an external iliac. 



112. The left internal iliac almost immediately gived 



