34 THE SKATE 



from the demibranch on the posterior side of the visceral 

 pouch (the anterior face of a gill-arch) and a small pre- 

 trematic from the demibranch on the anterior face of the 

 visceral pouch (posterior face of the gill-arch). The post- 

 trematic and the pre-trematic completely surround the 

 pouch, forming a loop around the gill cleft, meeting on the 

 ventral side. At the angle of the ventral and dorsal in- 

 ternal surfaces of the gill-arch a cross-branch is located 

 which unites the post-trematic of a gill-arch with the pre- 

 trematic which is parallel and just posterior to it in the 

 same arch. 



The anterior coronary arteries, the distribution of which 

 has been pointed out, carry aerated blood to the walls of 

 the heart. They arise from the extension around the fourth 

 and fifth gill-slits of certain vessels from the ventral ends 

 of the loops. The posterior coronary arteries are distri- 

 buted along the sinus venosus and originate from a branch 

 of the subclavian which will be encountered later. 



The common carotid originates at the junction of the 

 post-trematic and the pre-trematic from the most anterior 

 gill pouch as it forms the most anterior efferent artery. 

 Follow it forward. It bends slightly towards the median 

 line and just lateral to the anterior margin of the spiracle 

 it bifurcates, forming the external carotid which continues 

 towards the outside, and the internal carotid which con- 

 tinues towards the median line, where it meets with its fel- 

 low from the other side and passes through the cartilage 

 to the brain. 



There is an hyoidean artery which arises at about the 

 middle of the pre-trematic of the most anterior efferent 

 branchial artery. Follow it forward and laterally by cut- 

 ting away the cartilage behind which it passes. It goes to 

 the rudimentary gill of the spiracle and the adjacent mus- 

 cles. The branches to the spiracle reunite, forming the 



