ANIMAL STRUCTURE AND FUNCTION. 7 



Fat is stored in various situations to form cushions, to round off Fat. 

 corners, and to serve as a reserve of food if required. 



Noicrishmeilt is brought to the whole of these structures by the blood Use of 

 constantly circulating through the blood-vessels^ which are of three Blood; 

 varieties, arteries^ veins^ and capillaries. 



Arteries are muscular, thick-walled elastic tubes which, springing Arteries, 

 from the heart, ramify through the entire body. Gradually diminishing in 

 calibre as their length increases, they reach and supply every organ and 

 structure, finally becoming merged in the capillaries. 



Capillaries are, as their name denotes, the finest, thinnest, hair-like Capil- 

 tubes imaginable. They permeate every nook and cranny of the system, laries. 

 so that the merest pin prick will draw blood, z>., wound several of them. 



Veitis are the continuation of the capillaries on the return journey Veins, 

 towards the heart, approaching which they increase in size, receiving fresh 

 tributaries from all parts, till they empty themselves into that organ. 

 Here and there in the course of a vein, slight enlargement of its calibre 

 may be noticed, due to the presence of valves which prevent the blood 

 flowing backwards. A valve consists of two or three little skin-like 

 pouches with their openings towards the heart ; so long as the blood 

 flows in that direction they lie flat against the side of the vessel and do 

 not obstruct it, but if there is any back wash it automatically fills them, 

 causing them to bulge into the vein and completely block it. Their 

 action may be well observed by pressing the blood to and fro in the veins 

 on the back of one's own hands. Veins are thinner and are less elastic 

 than arteries, but are capable of containing more blood when distended. 



The Blood consists of a straw-coloured fluid, in which an infinite The blood, 

 number of minute bodies {corpuscles) are suspended. During life and 

 when freshly drawn from the body, blood is quite fluid, but rapidly 

 solidifies {clots) on exposure to the air. The blood co7piiscles are of two 

 varieties, red and white. Red corpuscles are very numerous, give the 

 blood its colour, are shaped like a quoit with a solid centre, and measure 

 about 3^ of an inch in diameter. Their peculiar power of absorbing 

 oxygen makes them important factors in questions of ventilation and 

 breathing. White corpuscles are comparatively few in numbers as 

 compared with the red variety, are larger, have the power of moving, 

 and are engaged in the duty of attacking and destroying any harmful 

 germs which attempt to invade the body. 



The Heart is a hollow, cone-shaped organ, composed entirely of The heart, 

 muscle. It is situated in the centre of the chest with its point directed 

 downwards and slightly inclined to the left. Internally it is divided into 

 four compartments, those on the left side being thicker walled and 



