STRUCTURE OF THE LUNGS OF MAMMALS. 55 



of the cartilaginous plates, where their growth is restricted, 

 and they consequently assume a flat cake-like form. In the 

 smaller bronchia they only occur between the cartilages, and 

 with the further division of these tubes become continually 

 smaller, until, with the disappearance of the cartilages, they 

 also entirely cease. From each of these glands a straight duct 

 lined with columnar epithelium, which especially in old people 

 may present at some part of its course a flask-like dilatation, 

 leads to the free internal surface of the bronchial tube, where it 

 opens by a trumpet-like orifice. 



To the external fibrous layer succeeds the muscular layer, 

 consisting of compact circularly arranged fasciculi of smooth 

 muscular tissue. Although this layer, on account of its present- 

 ing rounded sections of isolated circular fasciculi, cannot be 

 described as a tube with smooth and even parietes, yet the 

 muscular bundles are applied so closely to one another, and 

 communicate so frequently in a plexiform manner, that upon 

 the whole a continuous- layer is produced, the thickness of 

 which is usually proportionate to the diameter of the bronchial 

 tube. In those parts of the largest bronchi in the Horse which 

 are free from cartilage, its thickness amounts to about 0'5 of a 

 millimeter; in Man, O3; in the Dog, 0'2 O'l ; and in the Rat, 

 to 0*005 of a millimeter. In the bronchia of Man not exceed- 

 ing four millimeters in diameter its thickness amounts to O'l 

 of a millimeter ; in those of two millimeters' diameter, to 0'05 

 of a millimeter. Beneath the cartilages the muscular layer is 

 usually somewhat thinner. 



In opposition to the two layers above described, which in 

 transverse sections of the bronchia present the form of 

 annular zones of tolerably equable thickness, the immediately 

 succeeding internal fibrous layer offers on the same view a 

 regular alternation of thick and very thin parts, giving a 

 sinuous outline to the inner surface. This is due to the pre- 

 sence of from fourteen to twenty longitudinal elevations whose 

 height is dependent on the degree of development of the whole 

 layer, which again stands in a certain relation to the diameter 

 of the bronchia. Thick longitudinal elastic fibres constitute 

 the most characteristic . and essential elements of this layer; 

 they, however, do not form a stratum of uniform thickness, 



