22 ZOOLOGY. 



The student has probably already heard these terms 

 " artery " and " vein " employed, and very possibly is 

 accustomed to associate them with a difference in the 

 character of the blood they contain. It is well therefore 

 to insist at once on the point that the differences between 

 arteries and veins have nothing wJiatever to do with the com- 

 position of the blood they contain : they are connected solely 

 with the different conditions of pressure in the blood. 



5. Arteries contain blood flowing away from the heart 

 under the very great pressure caused by the powerful 

 pumping of that organ. Hence they have thick and highly 

 elastic walls, which do not allow any of the pressure to be 

 wasted in a mere distension of the artery : though distended 

 by the first shock of the heart's beat, the elastic recoil 

 follows, giving rise to the phenomenon known as the pulse. 

 Further, since the relative sectional area or calibre of the 

 arteries determines the proportion of the blood that shall 

 flow along them respectively, and since it is important that 

 these proportions should vary according to varying needs, it 

 is not surprising to find in the arterial walls a means of 

 adjustment in the form of a muscular layer which by con- 

 tracting in different degrees can alter the diameter of the 

 tube. When the rabbit is killed, there is no longer any 

 heart-pumping to act against the elasticity of the arterial 

 walls, and so the calibre is at a minimum, and the arteries 

 having thick walls and little blood remaining in them, 

 appear white (with a bluish or pinkish tinge) with a thin 

 reddish streak in the middle. Further, as the consequences 

 of a cut artery in a living animal are more dangerous than 

 those of a cut vein, we almost always find the large arteries 

 deeper- seated than the large veins. 



6. Veins. In veins, on the other hand, during life, 

 the pressure is low and variable. The blood is not pumped 

 into them, for they are separated from the elastic-walled 

 arteries by a sponge-like network of permeable -walled 

 capillaries, through which pressure-variations cannot be 

 transmitted. Hence the walls are thin, scarcely elastic, 

 and readily distended. Through these thin walls external 



