108 CHAPTER 10. 



thus obstruction to the circulation is guarded against. For somewhat 

 similar reasons arteries, when near the surface, such as those of the lips, 

 ears and nostrils, generally take a serpentine course. The extreme rami- 

 fications of the arteries form the arterial capillaries. They are infinitely 

 numerous. In them the outward or arterial circulation ends, and the 

 venous or return circulation commences. 



Arteries frequently anastomose with each other. By this provision of 

 nature a part still receives a supply, generally a sufficient supply of blood 

 for its nutriment from the collateral circulation, even though the arteries 

 which more immediately supply it may happen to be divided. 



Arteries have three coats, connected together by areolar tissue, all of 

 which are to a certain degree elastic ; namely, an outer coat of connective 

 tissue mixed with elastic fibres, a middle muscular and elastic coat, and 

 an inner cuticular coat. On account of the elasticity of its coats an 

 artery expands or contracts according to the amount of blood circulating 

 in it. Hence, in a certain sense an artery may be said to be always full. 

 The coats of the arteries are themselves supplied with blood by a minute 

 system of vessels called the vasa vasorum. 



An artery, when opened, generally continues to bleed freely on account 

 of the force of the circulation running through it, and the blood is 

 forced out in jerks at each contraction of the heart. Hence it is difficult 

 to stanch its bleeding. In order to do so, it is often necessary to take it 

 up and apply a ligature at the point of injury, or a compress, or acupres- 

 sure, or torsion, or styptics. In some cases, where the artery is only 

 partly divided, it may be possible to stop the bleeding by dividing it 

 completely ; because when it is cut right across, it contracts and retracts 

 within its sheath on account of the elastic nature of its coats. The Blood 

 which has escaped, forms a clot at the orifice, and another clot is formed 

 inside the vessel extending from the orifice to the next collateral branch. 

 The clot is conical in shape, and acts as a plug in preventing the fluid 

 blood from escaping. Cautery, either actual or potential, is also useful 

 as a means of stopping the flow of blood. There is not, however, much 

 fear of an animal bleeding to death from the effect of even a severe wound 

 of an artery, because the action of the heart is lowered by any consider- 

 able escape of blood, and the force of the circulation is thereby dimin- 

 ished. Arterial haemorrhage causes greater constitutional disturbance 

 than venous. 



209. The Veins. 



The veins are vessels which convey back to the heart the blood dis- 

 tributed by the arteries to different parts of the body. Veins are usually 

 more numerous and larger than their corresponding arteries. In most 

 cases there are as many as three veins to each artery. 



Veins are divided into superficial and deep-seated. The former are 

 found immediately below the skin, and hence are sometimes termed sub- 

 cutaneous ; whilst the latter generally accompany the deep-seated 

 arteries. Veins like arteries frequently anastomose. Veins have their 



