240 AN INTRODUCTION TO ZOOLOGY 



wall of the capsule, with the large orbital sinus. The latter is a 

 large expansion occupying the whole of the cavity of the orbit not 

 taken up by the eyeball and its muscles, and it receives towards its 

 antero-mesial end a small anterior cerebral vein coming through 

 the cranial wall from the front end of the brain. The two orbital 

 sinuses communicate with one another by means of an interorbital 

 vein which runs in the basicranial cartilage just behind the level of 

 the pituitary body and opens into the orbit by a foramen just in 

 front of and below the foramen for the main branches of the fifth 

 and seventh and the sixth cranial nerves. The orbital sinus is 

 continued backwards as the post-orbital sinus lying in the gutter- 

 like post- orbital groove which, as we have seen, lies on the side of 

 the auditory capsule below the ridge marking the horizontal semi- 

 circular canal and the articular surface for the hyomandibular 

 cartilage. In its course along the post-orbital groove the sinus 

 receives the posterior cerebral vein, an important vessel leaving the 

 cranial cavity in company with the tenth cranial nerve. Behind 

 the skull the venous trunk enlarges to form the anterior cardinal 

 sinus, a large irregular sinus situated along the internal dorsal ends 

 of the gill clefts. The pharyngeo-branchial cartilages project into 

 its floor, and the branchial branches of the vagus nerve pass freely 

 through its cavity. At its posterior end behind the last gill cleft 

 this sinus narrows considerably, and opens into the posterior cardinal 

 sinus through an opening guarded by a valve. 



A small vessel, the inferior jugular sinus, commences just behind 

 the symphysis of the lower jaws and runs backwards to the level 

 of the thyroid gland, where it communicates with its fellow by an 

 irregular anastomosis that surrounds the gland. From this point 

 it enlarges and becomes irregular, passing along the inner ventral 

 ends of the gill clefts and then along the pericardium, finally opening 

 into the ductus Cuvieri by a common opening with the sub-clavian 

 vein just outside the limits of the pericardial space. 



At the level of the thyroid gland a well-marked hyoidean sinus 

 leaves the side of the inferior jugular and passes out laterally in a 

 shallow groove in the external side of the hyomandibular cartilage. 

 It runs round parallel with the first gill cleft to open into the hinder 

 end of the post- orbital sinus, thus putting the dorsal and ventral 

 vessels in communication. 



Posterior Veins. 



The blood is brought back from the tail by the caudal vein, 

 which runs in the haemal canal accompanying and ventral to the 

 caudal artery. It leaves the canal and divides into two renal 

 portal veins that pass up the dorso-lateral edges of the kidneys, to 



