281 



RESPIRATION. 



surface is destitute of cilia. 

 Fig. 234. 



In the frog, as in 



of the internal lining. It is a curious fact which forms a thin pavement coating. This 

 that the exterior of the lung should be desti- ~ p ' ' " "" * ' " 



tute of cilia, while they should be present on 

 that of the liver in the newt. They are, how- 

 ever, on this last organ, limited to the margins 

 Nothing is more easy than to exhibit the living 

 circulation in the lung of the newt. The 



Fig. 233. 



A. small piece coinciding with the true capillary or 

 respiratory area from the lung of the Newt, viewed 

 by transmitted light tinder a high power. The 

 blood-corpuscles in streams are seen in the spaces 

 between the islets a, a, a. Only these parts are 

 in focus. The hyaline epithelium covering the near 

 face of the picture is out of sight. The eye looks 

 beyond it. In this fresh uninjected state the blood- 

 channels' do not appear to be bounded by separate 

 and independent parietes. (Original.) 

 a, a, a, are parenchymatous islets occupying the 



meshes of the capillary rete. They are composed 



of cells carrying nuclei and granules. 



internal bore of the vessel viewed by trans- 

 mitted light is much greater than the long 

 diameter of the red corpuscles. The meshes 

 (ff, ) are mere points. The scene is one 

 thick, rich, surpassingly beautiful network of 

 moving blood. 



In the frogs and toads the lungs consist of 

 two large, short, and broad, slightly cancellated 

 shining bags. They are situated on either 

 side of the spine, at the roof of the abdominal 

 cavity. They are remarkably elastic, like 

 those of the newt. They are capable of 

 slowly expelling their contents even after the 

 removal of the abdominal walls, and of draw- 

 ing themselves up into little hard balls on 

 either side of the pharynx. They exhibit well 

 the living circulation. The glottidean aperture 

 communicates directly with the interior of 

 the organ. There is, therefore, no trachea. 

 The orifice of the glottis is surrounded by 

 rudimentary cartilages somewhat further de- 

 veloped. The mechanism of breathing is the 

 same in the frog and toad, in which, like the 

 newt, the thoracic ribs are wanting, as in the 

 salamanders. The steps of the process are, 

 however, better studied in the frog. The 

 outer surface of the lung in the frog is closely 

 invested with peritoneum, the epithelium of 



Heart, vessels, and lungs of the Frog. 



the newt, the edges of the liver are fringed with 

 motile cilia. The pulmonary artery (see art. 

 CIRCULATION), derived from the aorta, pro- 

 ceeds along the outer side of the lung. It lies 

 immediately underneath the peritoneal epi- 

 thelium. The very reverse course is taken 

 by the large venous trunk on the opposite 

 side. This lies in immediate contact with 

 the internal or mucous surface. By this ar- 

 rangement the contact of the blood with the 

 air is prolonged. The contributory branches 

 of the vein course along the free internal 

 edges of the septa bounding the cells. The 

 branches of the artery occupy the opposed 

 fixed borders of the same septa. The flat 

 surfaces, or sides of the cells, being the areas 

 dividing the arteries and veins, are the scenes 

 of the capillary segments. To this rule, of 

 course, the eye, by close scrutiny, may detect 

 many exceptions. By this distribution of 

 parts, every spot of the internal superficies 

 is functionally utilised. The ciliary epithelium 

 is limited, in its distribution, to the margins 

 of the cells and the lines of the larger vessels. 

 The true capillary areas whereon alone respi- 

 ration actively proceeds are covered only by a 

 hyaline epithelium, the cells of which can only 

 be distinguished by their outlines {fg. 232. 

 c, c). The ciliated tracts, according to the 

 manner already described in the lung of the 

 newt, terminate by abrupt borders. The epi- 

 thelial cell only is continued over the capillary 

 areas. There prevails an average uniformity in 

 the dimensions of these areas. Each particle of 

 blood, therefore, in its transit from the artery 

 to the vein across this area, is exposed, for 

 the same period of time, to the influence of 

 the air. In the lung of the frog and toad the 

 septa support two layers of reticulate vessels, 



