222 COMPARATIVE ANATOMY 



mainder of it is thin, and is only prevented from being 

 flaccid by the cartilaginous gill-rays which support it, as the 

 silk of an umbrella is supported by the wires. The gill-rays 

 are covered over by a very thin sheet of interbranchial muscle, 

 and the blood-vessels carrying blood to and from the demi- 

 branchs run in the partition. The gill-pouches are. compressed 

 antero-posteriorly and laid back so as to overlap one another 

 to a considerable extent, and the external apertures are of 

 much less vertical height than the cavities of the pouches. 

 The outer edges of the posterior walls of the first, second, and 

 third pouches are strengthened by curved cartilaginous rods, 

 reaching from the lower sides of the external gill-slits nearly 

 to the mid-ventral line. These are the inferior extra-branchial 

 cartilages, and the upper edges of the posterior walls of the 

 first, second, third, and fourth pouches are similarly strengthened 

 by superior extra-branchial cartilages. Each demi-branch is 

 formed by numerous vascular folds of the mucous membrane 

 radiating outwards from the visceral arch and extending over 

 about two-thirds of the partition wall. A partition with its 

 demi-branch and extra-branchial cartilages is shown in fig. 5 3 A. 

 The whole branchial apparatus is covered over by a superficial 

 sheet of muscle, the constrictor superficialis branchiarum, 

 lying immediately below the skin. 



The gills are respiratory organs in whose capillaries the 

 blood is separated from the water by such thin membranes 

 that an exchange of gases is readily effected between the 

 blood and the air dissolved in the water. It is clear, then, 

 that the blood coming to the gills is what is called "venous" 

 blood, rich in carbonic acid and poor in oxygen, while the 

 blood leaving the gills is what is called " arterial" blood, richer 

 in oxygen and poorer in carbonic acid gas after its passage 

 through the gill capillaries. Notwithstanding this difference 

 in their contents, the blood-vessels bringing blood to the gills 

 are to be regarded as arteries as much as tho.se carrying blood 

 away from them, for, as has been explained in connection with 

 the frog (vol. i. p. 49), arteries are vessels carrying blood from 

 the heart, veins are vessels bringing blood back to the heart. 



The heart of the dogfish lies in the triangular pericardial 

 space already described as lying below the basi-branchial 

 cartilage and above the forward projection of the pectoral 

 girdle. 



