62 



STRUCTURE OF THE AIR-CELLS. 



corpuscles. This is surrounded by numerous fine elastic fibres, which give to the pulmonary 

 parenchyma its well-marked elastic characters (tig. 132, e, c). These fibres often bifurcate, and 

 are arranged with reference to the alveolar wall. They are very resistant, and in some cases of 

 lung disease may be recognised in the sputum. A few non-striped muscular fibres exist in the 

 delicate connective-tissue between adjoining air-vesicles. These muscular fibres sometimes 

 become greatly developed in certain diseases (Arnold, W. Stirling). The air-cells are lined by 

 two kinds of cells (1) large, transparent, clear polygonal (nucleated ?) squames or placoids 

 (22-45 n) lying over and between the capillaries in the alveolar wall (fig. 131, a) ; (2) small 

 irregular "granular " nucleated cells (7-15 /*) arranged singly or in groups (two or three) in the 

 interstices between the capillaries. They are well seen in a cat's lung (fig. 131, d). [When acted 



on with nitrate of silver the cement- 

 substance bounding the clear cells is 

 stained, but the small cells become of a 

 uniform brown granular appearance, so 

 that they are readily recognised. Small 

 holes or * pseudo-stomata " seem to 

 exist in the cement-substance, and are 

 most obvious in distended alveoli. They 

 open into the lymph-canalicular system 

 of the alveolar wall (Klein), and through 

 them the lymph-corpuscles, which are 

 always to be found on the surface of 

 the air-vesicles, migrate, and carry with 

 them into the lymphatics particles of 

 carbon derived from the air.] In the 

 alveolar walls is a very dense plexus of 

 fine capillaries (fig. 132, c), which lie 

 more towards the cavity of the air-ves- 

 icle, being covered only by the epithelial 

 lining of the air-cells. Between two 

 adjacent alveoli there is only a single 

 layer of capillaries (man), and on the 

 boundary-line between two air-cells the 

 course of the capillaries is twisted, thus 



f>rojecting sometimes into the one alveo- 

 us, sometimes into the other. 

 [The number of alveoli is stated to 

 Air-vesicles injected with silver nitrate, a, outlines De about 725 millions, a result obtained 

 of squamous epithelium ; b, alveolar wall ; c, young by measuring the size of the air-vesicles 

 epithelium cell ; d, aggregation of young epithelial and ascertaining the amount of air in 

 cells germinating. the lung after an ordinary inspiration, 



determining how much of this air is in 

 the air-vesicles and bronchi respectively. The superficial area of the air-vesicles is about 90 

 square metres, or 100 times greater than the surface of the body ("8 to *9 sq. metre).] 



The Blood-vessels of the lung belong to two different systems : (A) Pulmonary vessels 

 (lesser circulation). The branches of the pulmonary artery accompany the bronchi and are 

 closely applied to them. [As they proceed they branch, but the branches do not anastomose, 

 and ultimately they terminate in small arterioles, which supply several adjacent alveoli, each 

 arteriole splitting up into capillaries for several air-cells (fig. 132, v, c). An efferent vein usually 

 arises at the opposite side of the air-cells, and carries away the purified blood from the capillaries. 

 In their course these veins unite to form the pulmonary veins, which, again, are joined in their 

 course by a few small bronchial veins. The veins usually anastomose in the earlier part of their 

 course, whilst the corresponding arteries do not.] Although the capillary plexus is very fine and 

 dense, its sectional area is less than the sectional area of the systemic capillaries, so that the 

 blood-stream in the pulmonary capillaries must be more rapid than that in the capillaries of the 

 body generally. The pulmonary veins, unlike veins generally, are collectively narrower than 

 the pulmonary artery (water is given off in the lung), and they have no valves. [The pulmonary 

 artery contains venous blood, and the pulmonary veins pure or arterial blood], 



(B) The bronchial vessels represent the nutrient system of the lungs. They (1-3) arise from 

 the aorta (or intercostal arteries) and accompany the bronchi without anastomosing with the 

 branches of the pulmonary artery. In their course they give branches to the lymphatic glands 

 at the hilum of the lung, to the walls of the large blood-vessels (vasa vasorum), the pulmonary 

 pleura, the bronchial walls, and the interlobular septa. The blood which issues from their 

 capillaries is returned partly by the pulmonary veins hence, any considerable interference 

 with the pulmonary circulation causes congestion of the bronchial mucous membrane, resulting 

 in a catarrhal condition of that membrane. The greater part of the blood is returned by the 

 bronchial veins, which open into the vena azygos, intercostal vein, or superior vena cava. The 



