228 COMPARATIVE ANATOMY 



pelvic fins and tlie renal arteries, a number of small paired 

 vessels supplying the excretory organs. 



The blood which has been carried to the various organs 

 and tissues of the body by the arteries is returned to the 

 heart by a system of venous channels which, for the most 

 part, have the form of wide irregular spaces rather than of 

 definite tubes, and are therefore called sinuses. The blood 

 from the head and branchial region is returned by a pair 

 of large sinuses lying between the upper ends of the gill- 

 pouches and the muscles of the body, known as the anterior 

 cardinal sinuses, and these are supplemented by a pair of 

 smaller channels running along the ventral ends of the gill- 

 pouches, known as the inferior jugular sinuses. They dis- 

 charge their contents on either side into a short transverse 

 passage, the ductus Cuvieri, which is really a short and 

 narrow lateral continuation of the sinus venosus lying in a 

 notch in the posterior border of the fifth cerato-branchial 

 cartilage. The anterior cardinal sinus of either side com- 

 municates in front with a large blood space surrounding the 

 eyeball, the orbital sinus, by means of a narrow passage 

 running in the post-orbital groove of the auditory region of 

 the skull. It also communicates with the jugular sinus by 

 a relatively wide hyoideain sinus, running in a groove on the 

 outer side of the hyomandibular cartilage and by smaller 

 passages running down the outer faces of the first four 

 branchial arches. Thus the blood from the head and 

 branchial region is returned direct to the heart by way of 

 the ductus Cuvieri and sinus venosus. 



The blood from the kidneys, the genital organs, the trunk 

 muscles of the abdominal region, the pectoral fins, and the 

 lateral line, is also returned direct to the heart through the 

 posterior cardinal sinuses, whose position and extent is shown 

 in fig. 51. They lie close alongside of one another in the 

 roof of the abdominal cavity near the middle line ; their 

 dorsal walls are closely adherent to the body wall, their ventral 

 walls are covered below by the peritoneal lining of the ab- 

 dominal cavity. They take their origin from the posterior 

 section of the kidneys, from which they receive blood through 

 numerous renal veins. Thence they run forward, separated 

 by a median partition, and expand into very wide thin-walled 

 sacs lying right and left of the oesophagus, and communicat- 



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