200 



OF THE BLOOD AXD CIRCTJLATIOX. 



teries becomes the dark blood of the veins ; and again, in the 

 cells of the respiratory organs, which the capillaries supply, 

 the dark venous blood is oxygenated, and restored to the 

 bright scarlet hue of the arterial blood. 



359. Where there are blood-vessels, in the lowest animals, 

 the blood is kept in motion by the occasional contraction of 

 some of the principal vessels, as in the worms. Insects have a 

 large vessel running along the back, furnished with valves so 

 arranged that, when the vessel contracts, the blood can flow 

 only towards the head, and being thence distributed to the 

 body, is returned again into the dorsal vessel (fig. 223), by 

 fissures at its sides. 



360. In all the higher animals there is a central organ, 

 the heart, which forces the blood through the arteries towards 

 the periphery, and receives it again on its return. The HEART is 

 a hollow muscular organ, of a conical form, which dilates and 

 contracts at regular intervals, independently of the will. It is 

 either a single cavity, or is divided by walls into two, three, or 

 four compartments, as seen in the following diagrams. These 

 modifications are important in their connection with the respi- 

 ratory organs, and indicate the higher or lower rank of an 



Fig. 217. 



Lesser circulation. 



Pulmonary artery. 



Right auricle. 



Heart. _ 



Vena cava. 

 Right ventricle. 



, -^-Pulmonary veins. 



Left auricle. 



Aorta. 



Xeft ventricle. 



Greater circulation. 



