400 ANCJIOLOGY. 



sometimes ty two veins, and generally by a nerve, all of which 

 may lie in the same sheath, the veins being usually more super- 

 -ficial. The coats of arteries are supplied with nutrient blood- 

 vessels, the vasa. vasorwm, which come from neighbouring vessels, 

 and ramify in the external tunic ; they can occasionally be traced 

 into the middle tunic. Arteries are also provided with absorbents, 

 which are only visible on the larger trunks ; and plentifully 

 supplied with nerves, chiefly by the sympathetic, \7hich form 

 plexuses on the larger vessels. The external coat is very tough, 

 while the middb and internal are elastic and brittle. On 

 ligaturing an artery, the internal and middle coats give way, 

 while the external one is left unbroken. From the power of 

 resistance possessed by the external coat, we are enabled to stop 

 haemorrhage by torsion ; the middle and internal coats giving 

 way, the external, which remains unbroken, forms a spiral ligature 

 round the others, so tight that it cannot be unravelled by twisting 

 in an opposite direction. 



CAPILLARIES. 



The smaller arteries terminate in a system of minute vessels — 

 the cajnllaries — which are interposed between the termination of 

 the arteries and the commencement of the veins, forming plexuses 

 which vary much in arrangement. Their average diameter is 

 about a^oVo*^ ^^ ^^ inch, varying in different textures, smallest 

 in the brain and raucous membrane of the intestines, largest in 

 the skin, in glands, and the interior of bones. All arteries do 

 not terminate in capillaries, an exception being in erectile tissue, 

 where the arteries end in cells or cavities placed at the origin of 

 the veins. 



PULMONARY ARTERY. 

 (Fig. 154./.) 



The pulmonary artery springs from the conus arteriosus of the 

 right ventricle, its origin being guarded by the sigmoid valves ; 

 it curves upwards and. backwards until it reaches the upper part 

 of the left auricle, where it divides into right and left branches, 

 which penetrate the substance -of their respective lungs, along 

 with the bronchi, ramifying in the substance along with the 

 bronchial tubes, and terminating by capillary branches, which 

 form a dense network on the walls of the air-cells ; the branches, 



