ORGANS OF RESPIRATION. 2gl 



shreds from a blood clot, or emboli in the blood, to 

 filter through and reach the left side of the heart by 

 the pulmonary veins. If so, these emboli are 

 quickly carried by the blood current around the 

 aorta and up the right carotid, as the latter is the 

 most direct route. This course takes them to the 

 right side of the brain, in which the capillaries are 

 narrow, and where the emboli lodge often with fatal 

 results. Such emboli or shreds of blood clots pri- 

 marily enter the venous system at the seat of a bone 

 fracture, or in the walls of the uterus after parturi- 

 tion, or from clots of blood anywhere in the system. 



The pulmonary veins carry blood from the pul- 

 monary capillary plexus. Each venous radicle 

 drains an area corresponding to several air cells or 

 alveoli. At first these small veins take an inde- 

 pendent course in the interlobular tissue, but after 

 they have attained a certain size they accompany 

 the arteries and the bronchi, and, as a rule, along the 

 lower and front aspect of the latter. At the root of 

 the lung there are formed two pulmonary veins on 

 each side which open separately into the left auricle. 

 The pulmonary veins have no valves, and unlike the 

 veins of other organs are more capacious than their 

 corresponding arteries. 



Lymphatics. The lymphatics of the lung are very 

 extensive and accompany the two blood systems. 

 We may therefore divide them into two sets, a 

 bronchial and an alveolar. The bronchial consists of 

 an elaborate, fine plexus that ramifies through the 

 mucosa and submucosa of the bronchial tubes. This 

 set anastomoses freely with a second plexus just ex- 



