786 ORGANS OF VOICE AND RESPIRATION. 



posed. Within the lungs the bronchial tubes are circular, not flattened, and 

 their constituent elements present the following peculiarities of structure. 



The cartilages are not imperfect rings, but consist of thin laminae, of varied 

 form and size, scattered irregularly along the sides of the tube, being most dis- 

 tinct at the points of division of the bronchi. They may be traced into ttibes, 

 the diameter of which is only one-fourth of a line. Beyond this point, the tubes 

 are wholly membranous. The fibrous coat, and longitudinal elastic fibres, are 

 continued into the smallest ramifications of the bronchi. The muscular coat is 

 disposed in the form of a continuous layer of annular fibres, which may be 

 traced upon the smallest bronchial tubes ; they consist of the unstriped variety 

 of muscular fibre. The mucous membrane lines the bronchi and its ramifica- 

 tions throughout, and is covered with columnar ciliated epithelium. 



According to the observations of Mr. Rainey, 1 the lobular bronchial tubes, 

 on entering the substance of the lobules, divide and subdivide from four to nine 

 times, according to the size of the lobule, continuing to diminish in size until 

 they attain a diameter of V n th to ? ' n th of an inch. They then become changed 

 in structure, losing their cylindrical form, and are continued onwards as irregular 

 passages (intercellular passages, Rainey air-sacs, Waters), through the sub- 

 stance of the lobule, their sides and extremities being closely covered by nume- 

 rous saccular dilatations, the air-cells. This arrangement resembles most closely 

 the naked eye appearances observed in the reticulated structure of the lung of 

 the tortoise, and other reptilia. Opinions have differed as to the existence of 

 communications or anastomoses between the intercellular passages, or air-sacs. 

 According to Dr. Waters, 2 these air- sacs, as he terms them, are arranged in 

 groups, or " lobulettes" of five or six, which spring from the terminal dilatation 

 of a single bronchial tube, but have no other communication with each other, 

 or with neighboring lobulettes, than that which is afforded by their common 

 connection with the bronchial tubes. 



The air-cells, or alveoli (Waters), are small, polyhedral, alveolar recesses, 

 separated from each other by thin septa, and communicating freely with the 

 intercellular passages or air-sacs. They are well seen on the surface of the lung, 

 and vary from 5 ^th to ^V^h of an inch in diameter; being largest on the surface, 

 at the thin borders, and at the apex; and smallest in the interior. 



At the termination of the bronchial tubes, in the intercellular passages, their 

 constituent elements become changed : their walls are formed by an interlacing 

 of the longitudinal elastic bundles with fibrous tissue ; the muscular fibres dis- 

 appear, and the mucous membrane becomes thin and delicate, and lined with a 

 layer of squamous epithelium. The latter membrane lines the air-cells, and 

 forms by its reduplications the septa intervening between them. 



The Pulmonary Artery conveys the venous blood to the lungs : it divides into 

 branches which accompany the bronchial tubes, and terminates in a dense capil- 

 lary network upon the walls of the intercellular passages and air-cells. From 

 this network, the radicles of the pulmonary veins arise; coalescing into large 

 branches they accompany the arteries, and return the blood, purified by its 

 passage through the capillaries, to the left auricle of the heart. In the lung, 

 the branches of the pulmonary artery are usually above and in front of a bron- 

 chial tube, the vein below. 



The Pulmonary Capillaries form plexuses which lie immediately beneath the 

 mucous membrane, on the walls and septa of the air-cells, and upon the walls 

 of the intercellular passages. In the septa between the cells, the capillary net- 

 work forms a single layer. The capillaries form a very minute network, the 

 meshes of which are smaller than the vessels themselves: 3 their walls are also 

 exceedingly thin. The vessels of neighboring lobules are distinct from each 



i Medico-Chirurgical Transactions, vol. xxviii. 1845. 



* "The Anatomy of the Human Lung" 1860, pp. 136 150. 



3 The meshes are only 0.002"' to 0.008'" in width, while the vessels are 0.003'" to 0.005'". 

 Kiilliker, Human Microscopic Anatomy. 



