. 



114 OF THE BLOOD 



other important products derived from blood, the removal of 

 effete particles and the substitution of new ones, and all 

 those changes by which the bright blood of the arteries be- 

 comes 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. 



235. 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 ves- 

 Fig. 84. 



sel contracts, the blood can 



flow only towards the head, and, being thence distributed to 

 the body, is returned again into the dorsal vessel, (Fig. 84,) 

 by fissures at its sides. 



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

 the heart, which forces the blood through the arteries to- 

 wards 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, independ- 

 ently 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 respiratory organs, and indicate 

 the higher or lower rank of an animal, as determined by the 

 quality of the blood distributed in those organs. 



237. In the mammals and birds the heart is divided by a 

 vertical partition into two cavities, each of which is again 

 divided into two compartments, one above the other, as seen 

 in the diagram, (Fig. 85.) The two upper cavities are called 



