THE RABBIT. 21 



in the presence of oxidizable compounds, to which it yields 

 up the oxygen. The variation in the colour of the rabbit's 

 blood is due to the relative proportions in which haemoglobin 

 and oxy-haemoglobin are present. 



3. Capillaries. The blood must, to fulfil its functions, 

 come into intimate contact with all the tissues of the body, 

 and at the same time be able to travel rapidly from one 

 tissue to another. These two necessities are effected by 

 the enclosure of the blood in a series of tubes or vessels, and 

 by the walls of certain of these vessels being so thin that 

 the plasma of the blood can diffuse through them and 

 bathe the tissues around. These thin-walled vessels are 

 called capillaries: they are of microscopic dimensions and 

 are very abundant in those tissues (such as muscles and 

 glands) where extensive metabolism occurs. On the other 

 hand, in tissues where metabolism is slower, especially if 

 the tissue itself is readily permeable, capillaries are few 

 and far between, or even absent altogether, as is the case 

 with cartilage, which, though it may attain a thickness of 

 perhaps a quarter of an inch, is sufficiently served by the 

 plasma exuding from the capillaries in the surrounding 

 tissue (perichondrium). So too the epidermis, or outer 

 layer of the skin, is devoid of capillaries, as are all the 

 tissues known as epithelia. (See chap, vi.) 



4. Arteries and Veins. The walls of the capillaries 

 consist of the tissue known as endothelium (chap, vi., 3), 

 and this forms a continuous lining to the whole of the 

 blood-vessels of the body, but in the capillaries alone is it 

 the only layer present. The capillaries in a given portion 

 of a tissue form a complex network through which the 

 blood rushes rapidly, as can easily be seen in the tail of 

 a living tadpole under the microscope. Observation soon 

 shows that blood is brought to this capillary network in one 

 vessel, and conveyed away through another. These two 

 vessels frequently run side by side ; usually they belong 

 respectively to the category of arteries and veins the 

 artery bringing blood to the capillary network and the vein 

 conveying it away. In certain cases both vessels are veins. 



