CHAP, viii.] ORGANS OF RESPIRATION AND SECRETION. 225 



Each bronchial tube, while still cylindrical, enters one of the 

 lobules and there ramifies, its ramifications ultimately dilating into 

 a larger passage called an infundibulum, the walls of which are 

 beset with numerous little sac-like dilatations, called air-cells, alveoli, 

 m pulmonary vesicles. These are naturally filled with air, and their 

 membranous walls are strengthened with elastic and some muscular 

 fibres, and beset with a multitude of delicate capillary vessels, which 

 expose the blood they contain to the action of the air in the alveoli. 

 The arteries (carrying the venous blood for oxygenation) end in 

 minute twigs, which surround the margins of the alveoli, whence 

 the capillary vessels extend inwards, and are subjected to the air on 

 both sides of the moist, delicate membrane in which they ramify. 

 The minute veins which issue from the capillaries, and which carry 

 arterial blood, do not run side by side with the arteries, but, pur- 

 suing a different course, frequently anastomose and increase in size 

 till they end in the great pulmonary veins, which proceed through 

 the roots of the lungs to the left auricle, as before described. 



Besides the pulmonary arteries (which bring blood for respiration), 

 the lungs have their own proper arteries and veins, which are 

 concerned in their nutrition. These are the bronchial arteries and 

 veins, and they are smaller than the other blood-vessels of the lungs 

 the pulmonary arteries and veins. 



The bronchial arteries are derived from the aorta, and follow the 

 divisions of the bronchi within the lung. The bronchial veins unite 

 together to pass out at the roots of the lungs. 



5. The MECHANISM or RESPIRATION has already been slightly 

 noticed, in the fifth chapter, in relation to the action of the 

 diaphragm and intercostal muscles, which serve, by their alternate 

 contraction and dilatation, to modify the capacity of the thorax. 

 The serrati postici muscles, by drawing backwards the ribs to which 

 the diaphragm is attached (at the very time that the other ribs are 

 being drawn forwards, and the diaphragm, by its contraction, 

 rendered less convex) aid in temporarily enlarging the thoracic 

 cavity, and so causing an influx of air into the lungs. In this action 

 the serrati ant id and scale ni also give aid by drawing the anterior 

 ribs forwards. 



The pumping action of the diaphragm is the .main agent in 

 respiration, the relaxation of its fibres allowing it to become convex 

 anteriorly, and so encroach upon the thoracic cavity air being 

 necessarily driven out of the lungs thereby. But the expulsion of 

 air in expiration is largely due to the highly- elastic nature of the 

 pulmonary structures (which has been already pointed out),* and to 

 the muscular contraction of the bronchi. The abdominal muscles 

 however are also called into play to effect a forcible expiration. 



* It is owing to this elasticity that if 

 a perforation be made in the wall of the 

 thorax, the lungs will contract greatly. 

 For by such injury the atmospheric pres- 

 sure within, becomes neutralized by 



atmospheric pressure on their outer sur- 

 face ; from which pressure, the un- 

 injured thoracic wall before protected 

 them. 



