22 Animal Life and Intelligence. 



the minute structure of the lungs ; suffice it to say that, 

 in the mammal, they contain a vast number of tubes, all 

 communicating eventually with the windpipe, and terminat- 

 ing in little expanded sacs or bagfl. Around these little 

 sacs courses the blood in a network of minute capillary 

 vessels, the walls of which are so thin and delicate that 

 the fluid they contain is only separated from the gas 

 within the sacs by a film of organic tissue. 



The blood is a colourless fluid, containing a great 

 number of round red blood-discs, which, from their minute 

 size and vast numbers, seem to stain it red. They may 

 be likened to a fleet of little boats, each capable of being 

 laden with a freight of oxygen gas, while the stream in 

 which they float is saturated with carbonic acid gas. This 

 latter escapes into the air-sacs as the fluid courses through 

 the delicate capillary tubes. 



Whither goes the oxygen ? Whence comes the carbonic 

 acid gas? The answer to these questions is found by 

 following the course of the blood-circulation. The pro- 

 pulsion of the blood throughout the body is effected by the 

 heart, an organ consisting, in mammals, of two receivers 

 (auricles) into which blood is poured, and two powerful 

 force-pumps (ventricles), supplied with blood from the 

 receivers and driving it through great arteries to various 

 parts of the body. There are valves between the receivers 

 and the force-pumps and at the commencement of the 

 great arterial vessels, which ensure the passage of the 

 blood in the right direction. The two receivers lie side by 

 side ; the two force-pumps form a single muscular mass ; 

 and all four are bound up into one organ; but there is, 

 during adult life, no direct communication between the 

 right and left receivers or the right and left force-pumps. 



Let us now follow the purified stream, with its oxygen- 

 laden blood-discs, as it leaves the capillary tubes of the 

 lungs. It generally collects, augmented by blood from 

 other similar vessels, into large veins, which pour their 

 contents into the left receiver. Thence it passes on into 

 the left force-pump, by which it is propelled, through a 



