13 



veins which follow the hyoid arch. At the outer end of the cuvierian 

 duct there is often a small opening on the anterior wall opposite the 

 mouth of the posterior cardinal vein. This leads into the anterior 

 cardinal vein. As mentioned before, in a slight majority of the 

 cases examined, the anterior cardinal vein opens into the posterior 

 cardinal vein, not directly into the cuvierian duct. If possible, pass 

 a bristle into the anterior cardinal. To follow the vein, and usually 

 this is the best way to find it, make a vertical longitudinal incision 

 upon the dorsal side of the neck, between the gill pouches and the 

 mass of muscle lying beside the vertebral column. This will open 

 the anterior cardinal, which is considerably expanded in this region, 

 and it may be traced from this point toward the heart and the head. 

 The anterior cardinal narrows suddenly in front of the anterior gill 

 pouch, and leads downward to the orbit, where it expands into the 

 orbital sinus surrounding the eyeball and its muscles. Trace the 

 anterior cardinal only as far as the opening into the orbital sinus at 

 this time. Veins from the anterior portion of the head and from 

 the brain can be followed when the dissection of the eye is under- 

 taken. 



Just back of the spiracle the anterior cardinal receives the hyoi- 

 dean vein., which passes ventrad along the base of the first demi- 

 branch and unites with the hyoidean of the opposite side. Ventrally, 

 it also communicates with the inferior jugular vein. 



The principal veins of the body have now been dissected with 

 the exception of the hepatic portal vein, which it is better to trace 

 after the arteries of the digestive tract have been studied. 



THE EFFERENT BRANCHIAL ARTERIES AND DORSAL AORTA. Com- 

 mencing at the mouth, cut through the floor of the pharynx close to 

 the left side of the ventral aorta and the heart. The cut should leave 

 the gill arches uninjured, and may be continued into the oesophagus. 



Examine the interior of the mouth and pharynx, observing par- 

 ticularly the form and arrangement of the teeth, the spiracular and 

 branchial clefts, the gill-rakers, and the character of the mucous 

 coat of the pharynx. 



Remove the skin from the roof of the pharynx. This exposes 

 four pairs of efferent branchial arteries bringing blood from the 

 gills and uniting in pairs to form the dorsal aorta. Follow each ves- 

 sel of the left side out to its gill-cleft. At the dorsal end of the gill- 

 cleft it divides into a large posterior and small anterior branch. 

 These respectively pass along the posterior and anterior demibranchs 

 of the gill pouch, receiving fine branches from the gill lamellae, 

 and finally unite again at the ventral end of the gill-pouch. Thus 

 a complete loop is formed around the branchial cleft. The posterior 

 branch of each efferent artery and the anterior branch of the suc- 

 ceeding one are united by several short vessels. The efferent artery 

 of the last demibranch possesses only these connections with the 

 branch next anterior to it, and none with the aorta directly. From 

 the ventral ends of the efferent loops small vessels pass toward the 

 mid-line to unite with a longitudinal artery, the hypobranchial ar- 

 tery, which will be traced farther a little later in the dissection. 



In Eugaleus the dorsal aorta extends forward beyond the union of the first 

 pair of efferent branchials and then divides into small right and left branches 

 which pass forward and outward to unite with the common carotid arteries. 



A common carotid artery leaves the dorsal end of each anterior 

 efferent branchial loop, passing forward and inward. At the level 

 of the spiracles it divides into external and internal carotids; the 

 internal carotid unites with its fellow of the opposite side and enters 

 the skull. The external carotid arteries run outward and forward 



