78 THE MAIN CURRENTS OF ZOOLOGY 



far-reaching consequences than appear on the sur- 

 face. It was not merely showing that the blood 

 moves in a circuit, it also opened up the entire ques- 

 tion of the part played by the blood in the animal 

 economy. This is basal to the understanding of 

 physiology. 



The tissues are bathed by the blood current. It is 

 the carrier of oxygen to the tissues of the body; into 

 its stream are diffused all the products of digestion, of 

 secretion, and of excretion; by its circulation it 

 distributes nourishment to the most remote parts of 

 the body and brings back the products of worn out 

 tissues for elimination by the kidneys, the lungs and 

 the skin. Accordingly, a proper conception of the 

 circulation was a necessary preliminary for the prog- 

 ress of physiology. 



The classic book of Harvey (Fig. 30) on the cir- 

 culation of the blood (De Motu Cordis et Sanguinis) 

 was published in 1628, but he had been giving the 

 substance of it in his university lectures since 1619. 



In the time of Harvey, however, physiology had 

 not fully emerged it was still intermingled with 

 medical studies. It was not till the following cen- 

 tury, through the work of Haller, that it took its 

 place as an independent subject to be pursued for its 



