556 STRUCTURE OF LUNG. [BOOK n. 



capillary surrounding the island to meet a similar extension of 

 another cell whose nucleus is placed in the next island. The 

 part of the cell in which the nucleus is placed, though thin, has 

 some little depth, but the part of the cell stretching over the 

 capillary is reduced to the merest film. Hence each island or 

 mesh is occupied by the nuclei and by the thicker parts of two, 

 three or more converging cells, while the capillary network sur- 

 rounding the island is separated from the interior of the lung by 

 the extremely thin flat expansion of cells belonging to that and 

 to the neighbouring islands. The blood passing through the 

 capillary is in consequence separated from the air in the lung by 

 nothing more than the capillary wall itself and a film, which has 

 not even the thickness of a flat epithelium cell but is only a 

 wing-like extension of a cell itself flat. The capillaries are in fact 

 imbedded as it were in the epithelial layer. By this means the 

 partition between the blood and the air is reduced to almost the 

 narrowest possible limits. Near the neck of the sac the network 

 becomes more open, and at the neck the peculiar epithelium just 

 described somewhat suddenly changes into a single layer of rather 

 short but otherwise ordinary columnar ciliated cells. 



The outer part of the connective tissue basis, away from the 

 epithelium, becoming somewhat looser in texture but still richly 

 provided with elastic fibres, contains besides the small arteries and 

 veins belonging to the capillary networks many small bundles of 

 plain muscular fibres, chiefly running in a circular or transverse 

 direction. Small branches of the vagus nerve pass to the lung, 

 running in company with the pulmonary veins ; connected with 

 these, towards the upper part of the lung, are numerous small 

 groups of nerve cells. The nerve fibres, which are chiefly non- 

 medullated, though medullated fibres are also present, end probably 

 in the muscular fibres or in the blood vessels. Branched pigment 

 cells are also present. 



316. The lung of the frog repeats, in structure, most of the 

 features of the newt's lung just described, but is more complicated. 

 The cavity of the sac, especially in its upper part, is broken up by 

 a number of partitions or septa projecting into the interior. Each 

 septum is a fold of the wall of the cavity, and consists of a middle 

 basis of connective tissue, covered on each side with epithelium. 

 From these primary septa start in a similar manner secondary 

 septa of a similar structure, projecting into the open chambers or 

 divisions of the whole sac, formed by the primary septa, and 

 dividing these into smaller open chambers; and many of these 

 secondary septa bear in a similar manner similar tertiary septa, 

 dividing the secondary chambers into tertiary chambers, or alveoli. 

 In this way, especially in its upper part, the cavity of the lung is 

 divided into a honeycomb of chambers or alveoli, the smaller or 

 tertiary alveoli opening into the secondary chambers, the secondary 

 into the primary, and the primary into the general cavity of the 



