802 SPECIAL PHYSIOLOGY. 



sue, which attaches their sides together, and is itself connected with a 

 general covering of areolar tissue, found upon the surface of the lung, 

 beneath the pleura, named the subpleural or subserous coat. Both 

 the interlobular and subserous tissues contain many elastic fibres. 



Fig. 111. Back view of the lungs and heart, with the air-tubes and. great bloodvessels attached. 1, tra- 

 chea or windpipe. 2, 2, lungs ; the left one partly dissected, to show the bronchial tubes and the pulmon- 

 ary arteries and veins branching in it. 3, heart. 4. left bronchus, entering at the root of the lung, be- 

 neath the arch of the aorta, which is seen cut across. 6. left pulmonary artery. 7, 8, left pulmonary 

 veins, entering, 5, the left auricle; beneath the right pulmonary veins is the inferior vena cava, cut 

 across. 9, back or under surface of the left ventricle of the heart. 



On entering the lung, each bronchus 'divides into an upper and 

 lower branch, one for each lobe ; but, on the right side, the lower 

 branch gives off a smaller one, which passes into the middle or third 

 lobe of that lung. Inside the lung, the bronchi continue to subdivide, 

 at obtuse angles, into smaller and smaller tubes, called bronchia; 

 these never coalesce again, but continue separate, after the manner of 

 the primary ducts of a gland, or of the branches of a tree. These 

 tubes generally divide dichotomously, or in twos, but sometimes three 

 bronchia proceed from a common trunk, or small lateral bronchia are 

 given off. The finest bronchia, forming the lobular bronchial tubes, 

 end in the lobules, as will presently be described. The combined 

 sectional area of the smaller bronchia is very much greater than that 

 of the larger bronchia or the trachea. 



Within the lung, the bronchia are round, and not flattened poste- 

 riorly, like the trachea and its primary divisions in the root of the lung. 

 Their constituent elements are similar to those of the trachea, but these 

 are here much modified. Thus, the cartilages, instead of being ar- 

 ranged in regular transverse slips, assume the form of angular or 

 polygonal plates of proportionate size, placed on all sides of the tubes. 

 At the angles of bifurcation of the tubes, spur-shaped pieces of cartilage 

 usually support them; minute flakes are found even in the smaller 



