318 THE LAEYNX, LUNGS, VOCAL SACS, ETC. 



surface of the organ ; when seen in section l (PL II, Fig. 208 A, .#) 

 these bands are found to be composed of well-developed involun- 

 tary muscular fibres. Between the larger bands are smaller bands 

 having a similar arrangement. From these networks of muscular 

 bands finer processes of muscular tissue pass peripherally towards 

 the surface of the organ, and are attached to the thin and incom- 

 plete muscular layer found in the wall of the lung (C). 



(2) The connective-tissue of the lungs is present in only small 

 quantity, but is still sufficient to fill in the spaces between the 

 various muscular bands and the surface of the lung, and to invest 

 the whole of that surface. There is thus formed a series of pits, 

 the mouths of which open into the general cavity of the lung, 

 while their bases are at the surface. Through this connective- 

 tissue course the blood-vessels, nerves, and lymphatics. It contains 

 numerous yellow elastic fibres. 



(3) The blood-vessels of the lungs. The pulmonary artery i 

 courses along the outer surface of the lung to the apex, giving 

 off, at right angles, lateral branches in the whole of its course ; 

 these show a tendency to be alternately larger and smaller. The 

 lateral branches divide and form a rich capillary network (T. 

 Hoffmann). 



The capillary network has very small meshes ; the diameter of a 

 given mesh being frequently less than that of the capillary bounding 

 it. The meshes are rounded or polygonal in shape. 



The pulmonary vein arises by lateral branches from this capillary 

 network ; the branches join, at right angles, the main vein, which 

 courses from the apex of the lung along its inner surface to the 

 root of the organ. 



(4) The epithelium of the lungs. Externally the lungs are 

 covered with a layer of endothelium derived from the peritoneum. 

 Internally the surface is covered with an epithelium which varies 

 considerably in different positions. 



On the free borders of the muscular trabeculae forming the 



1 See Plate II, Fig. 208. 



Two sections from the lung of Rana temporaria ; stained with borax carmine. G. H. 



I. The lung dilated (Hartnack, Oc. I, Syst. 3). 

 II. The lung contracted (Hartnack, Oc. I, Syst. 7). 

 A Band of muscle cut transversely. li Band of miucle cut longitudinally. 



6' Muscular layer of surface. 



