306 HISTORY OF THE HUMAN BODY 



afferent branchiate, which bring the blood directly from the 

 heart to the gills, and the efferent branchiate, which collect the 

 blood from the gill capillaries and unite to form the main 

 aorta. Since the blood is aerated in the capillary network of 

 the gills it follows that the blood coming from the heart 

 through the afferent vessels is impure, while that in the effer- 

 ent vessels is pure ; and since these latter unite to form the main 

 aorta, this vessel, the branches of which supply the entire body, 

 contains only aerated blood, while the heart is employed merely 

 to receive the venous blood which returns from all parts, and 

 to send it to the gills. This is the primitive type of vertebrate 

 circulation, and obtains not only in all fishes, but reappears as 

 the early form in all vertebrate embryos, thus proving its 

 fundamental character as an historic stage. 



The essential respiratory cavity is thus the entire pharynx, 

 through which a current of water is kept in constant flow by 

 being taken in at the mouth and exhaled through the gill- 

 slits; and while in earlier forms, as suggested by Amphioxus, 

 the capillaries lie in the unmodified pharyngeal wall in the 

 vicinity of the gill-slits, the selachians show a considerable ad- 

 vance by the formation of definitely localized organs, with a 

 Jarge increase of surface, and thus physiologically more effi- 

 cient. 



In other fishes this gill-system, which is essentially similar to 

 the foregoing, exhibits several secondary modifications, the 

 most apparent of which is the formation of a large gill-flap 

 (operculum), which arises in front of the most anterior slit 

 and extends backwards, and as the slits become closely ap- 

 proximated and are reduced in number to four, the operculum 

 becomes capable of closing entirely over them, meeting a ridge 

 of integument behind the last slit (Fig. 85, a). The current 

 of water is directed by rhythmic respiratory movements, which 

 consist of opening and closing the mouth and operculum, the 

 motions of the two alternating with each other. 



With the fishes true internal (endodermic) gills pass away, 

 but in the permanently aquatic salamanders and in all larval 

 amphibians one or more slits break through, usually two to 



