364 



OF RESPIRATION. 



shaped orifices. They cease with the cartilaginous plates between which 

 only they ultimately appear. The smaller bronchi, or those having a diame- 

 ter of about one-fiftieth of an inch, differ from the foregoing chiefly in the 

 thinness of their external fibrous coat, and the absence in it of either car- 

 tilages or mucous glands, whilst the columnar ciliated epithelium becomes 



FIG. 157. 



a 



Part of a transverse section of a bronchial tube from the Pig, having a diameter of about 1-GOth of 

 an inch, magnified 240 diameters, a, external fibrous layer; 6, muscular layer; c, internal fibrous 

 layer ; d, epithelial layer; /, one of the surrounding alveoli. 



lower or more flattened, and finally at the termination of the bronchia in 

 the alveolar passages the cells lose their cilia and the cup cells cease to ap- 

 pear. The bronchial tubes divide and subdivide, and at their termination 

 (a, Fig. 158), open into canals known as the alveolar passages, the entrance 

 of which is surrounded by a thick bundle of annular muscular fibre-cells 

 forming a kind of sphincter. 1 The alveolar passages, c, c, are rather wider 

 than the bronchia, from which they spring, and, after dividing dichotomously 

 at acute angles from two to four times, terminate caecally at a distance of from 

 ynth to Jgth of an inch from the extremity of each bronchial tube. In cou- 

 sequeuce of these ultimate processes gradually expanding towards their cttcal 

 terminations they have been termed infundibula. Both the alveolar passages 

 and the infundibula (air-sacs of Dr. Waters, which he describes rather dif- 

 ferently from Schulze as forming a group of from six to ten in number 

 clustered around the extremity of every bronchial tube) are beset with 

 numerous small polyhedric cavities with rounded angles and borders termed 

 the alveoli or air-cells (Fig. 158) ; from ten to twenty of each may be counted 

 on the interior of each iufuudibulum or air-sac. The length of the alveoli 

 of ordinary size are about equal to their breadth ; the total number has been 

 estimated approximatively at 1700 or 1800 millions, with a surface equivalent 

 to 200 square metres (Kiiss). 2 The foundation layer of the alveolar wall is 

 composed of a transparent structureless membrane, which in the thickest 

 parts exhibits a distinctly fibrous character, caused by the presence of nu- 

 merous clastic fibres. 3 The membrane is folded sharply at the margin of 

 the air-vesicles (Fig. 159, c), so as to form a very definite border to them. 

 The stronger septa divide oft' polyhedric portions of the alveolar parenchyma, 

 which are known as lobnli. The diameter of the Human air-cells is about 



1 Pundfldscli, Ccntnilblatt, 1872, p. 65. - Physiologic, 1873, p. 338. 



3 Hirschmann utid Chrzonsczczewsky, Virchow's Archiv, I860, p. 355. 



