328 HISTORY OF THE HUMAN BODY 



collected by one or more hepatic veins, lying more on the ven- 

 tral side of the liver, and is emptied into the sinus venosus. 

 Thus all the impure blood, through one channel or another, 

 finds its way into this most posterior chamber of the heart, 

 from which it passes in succession through atrium, ventricle, 

 and conus arteriosus, and finally into the gills, where it be- 

 comes aerated. 



It thus happens that the heart contains only impure or 

 venous blood, since it is not purified until it reaches the gills, 

 which suggests that the terms " arterial " and " venous," as 

 applied to pure and impure blood respectively, are not applica- 

 ble in the case of the lower vertebrates, and are much better 

 dropped, since they are often misleading. Furthermore, in 

 Amphibia and Reptilia these two kinds of blood are not sharply 

 defined, since both sorts are often allowed to mingle, forming 

 a mixed blood of varying degrees of purity. All confusion on 

 this point, however, may be avoided if the terms artery and 

 vein and their corresponding adjectives are used in their an- 

 atomical sense only, arteries being, as previously defined, those 

 vessels in which the blood flows from the heart, and veins those 

 in which the blood flows towards the heart. The physiological 

 distinction which designates pure blood as arterial and impure 

 blood as venous is taken from its condition in the two sets of 

 vessels in birds and mammals, and even here in the case of 

 the pulmonary system the conditions are reversed and physio- 

 logically arterial blood flows in the veins, and physiologically 

 venous blood in the arteries. 



The history of the arterial arches is shown in synoptical 

 form by the accompanying series of diagrams (Fig. 91), which 

 present the facts as deduced from the combined study of both 

 the adult anatomy and embryological development of repre- 

 sentatives of each Class of vertebrates. The diagrams repre- 

 sent the adult conditions in each case, the relationship being 

 morphologically interpreted by the help of the development. 



There are typically six pairs of arterial arches, which lie 

 along the sides of the pharynx and extend from a ventral vessel 

 that proceeds directly from the heart to a dorsal one that col- 



