PLANS OF CIRCULATION. 209 



ration ; and in the nutrient system, towards b, 

 representing the aggregate of the other organs of 

 the body.* 



Besides the distinction now drawn between the 

 two systems, founded on their general function as 

 being nutrient and oxygenising, (represented in the 

 diagram as occupying the right and left sides 

 respectively), it will often be convenient to consider 

 the circuit of the blood as being divided into two 

 portions, according as they relate to the organs 

 which are traversed ; namely, the general system 

 of the body, and the respiratory apparatus. The 

 first of these may be designated the system ic, and 

 the second the respiratory circulation: in the dia- 

 gram, the former is represented as occupying the 

 lower, and the latter the upper half of the figure. 



The systemic system comprehends the nutrient 

 arteries, the main trunk of which is called the 

 aorta, and the veins, belonging to the oxygenising 

 system, the trunks of which are termed the vence 

 cavce. The respiratory system is either pulmonary 

 or branchial, according as the organs of respiration 

 are adapted to the admission of air in its gaseous 

 state into lungs or other pulmonary cavities, or of 

 water containing air, in which case they constitute 

 branchice, or gills ; and it comprehends the set of 

 arteries (which are either pulmonary or branchial), 



* Besides the oxygenising system of vessels, the object of which 

 is the elimination of carbon, tliere exist also, in most animals, two 

 other vascular systems of less extent than the former, and directed 

 to special organs of secretion ; namely, the liver, for the elimination 

 of redundant hydrogen, and the kidneys, for the elimination of 

 redundant nitrogen. The first of these constitutes the hepatic, and 

 the latter the venal system. They will be farther noticed here- 

 after. 



VOL. II. P 



