18 GUIDES FOR VERTEBRATE DISSECTION 



Now lay open the lateral portions of the sinus venosus (these 

 parts are morphologically the Cuvierian ducts), wash out the 

 contents, and find the following vessels entering them, in addi- 

 tion to the lateral veins already seen: (1) at the posterior dorsal 

 surface a pair of postcardinal veins each of which expands almost 

 immediately (dorsal to the base of the liver) into a cardinal sinus 

 and then continues back, just to one side of the median line, along 

 the mesonephros. Lay open the sinus and vein. In the sinus see 

 the bands (trabeculse) strengthening the walls, and farther back 

 the openings of veins coming from the mesonephros. Do you find 

 any communications between the postcardinals of the two sides? 

 (2) Opposite the opening of each cardinal sinus the opening of 

 a jugular sinus (a greatly enlarged jugular vein). Trace this for- 

 ward with probe and scissors on one side, noticing that it contracts 

 and then enlarges to form an orbital sinus, partly surrounding 

 the eyes. 



Add jugulars and postcardinals to the- sketch of the venous 

 system. 



Follow the aorta backwards from the union of the efferent 

 branchial arteries, noting the connection with it of the various 

 arteries (coeliac axis, mesenteries, etc.) left in the mesentery 

 which supplied the alimentary tract. Also notice small vessels 

 (renal arteries) passing to the mesonephros, as well as others 

 (intercostal arteries) distributed in pairs to the body wall. Be- 

 sides these trace a subclavian artery on either side arising from the 

 last efferent branchial and passing into the pectoral fin. In its 

 course the subclavian gives off a small lateral artery which runs 

 backward in the body wall, ventral to the lateral vein. Parallel- 

 ing the subclavian artery in the more lateral part of its course is 

 a subclavian vein which can be traced medially into the sinus 

 venosus. 



Connect the dorsal aorta, subclavians, and the roots of the 

 arteries of the digestive tract -with the sketches already made 

 and insert the renals, intercostals, and laterals as well. 



Cut off the tail about two inches behind the anus and in the 

 cut-surface make out on either side, immediately beneath the 

 skin, the small canal of the lateral-line system. The mass of the 

 tail is made up of muscles on the right and left sides, and each 

 side is divided into dorsal epaxial and ventral hypaxial parts. 

 In the centre of the tail is the vertebral column, consisting of a 



