XROXUU1 AND LUNGS. 211 



of the alveoli, and gives out in turn gases which are to be 

 eliminated. The gases pass through the vascular epithelium, 

 the connective tissue between the vessels and the wall, the basal 

 membrane, and the respiratory epithelium. 



The arterial blood supply to the lung is accomplished by 

 the bronchial arteries. These break up into small branches, 

 which supply the bronchi, the interlobular connective tissue, 

 and the walls of the pulmonary vessels. There are numerous 

 anastomoses between the bronchial and pulmonary systems of 

 blood-vessels. A part of the blood of the bronchial arteries 

 thus leaves the lungs through the bronchial veins, and a part 

 through the pulmonary veins. 



The branches of the pulmonary artery follow the bronchus 

 to a point beyond the terminal bronchus. As has been 

 described by W. S. Miller, the branches at this point divide 

 to send twigs to each atrium. From these a capillary network 

 is formed, which surrounds the air sacs and air cells. On the 

 peripheral side of the air sacs and air cells the capillaries 

 gather to form the veins, which remain at the outside of the 

 lobule, as shown schematically in Fig. 161. The network 

 which is shown in Fig. 162 is the richest capillary plexus in 

 the body. It is thus seen that the lobule of the lung forms 

 also a blood vascular unit, with the artery in the centre and 

 the veins at the periphery. An exception to this is formed by 

 two small veins arising from near the end of the terminal 

 bronchus. 



Lymphatics in the lung and bronchi have been studied by 

 Miller. In the bronchus the lymph- vessels form a network 

 which extends as far as the end of the terminal bronchus. 

 Here branches are sent to the pulmonary artery, to the two 

 small veins in this region, and to the veins that run to the 

 pleura, 



Nerves. Nerve fibres follow the bronchi into the lung sub- 

 stance. These consist of both medullated and non-medullated 

 fibres. The sympathetic fibres show small ganglia in their 

 course. These nerves innervate the muscles and mucous mem- 

 branes of the bronchi, and also the walls of the blood-vessels. 



