LUNGS. 259 



is seen in the lungs of Testudo, in which a single thin-walled process extends 

 backwards to the" pel vie region. These processes seeih to foreshadow a con- 

 dition which reaches its highest development in Birds. 



A uniform ground-plan is to be observed in the arrangement 

 of the mtra-pulmonary bronchial system through the whole series 

 of the Amniota, from Crocodiles onwards. A continuation of 

 the bronchus, which is almost straight, always passes through the 

 lung to its posterior end. This may be called the main bronchus; 

 from it a series of lateral bronchi arise. The important and 

 typical relations of the latter to the main trunk of the pulmonary 

 artery and vein in Mammals, will be described later (p. 263) : it is 

 not yet known whether a similar arrangement obtains in Chelonians, 

 Crocodiles, and Birds. 



Birds. The respiratory apparatus of Birds presents so many 

 remarkable peculiarities, both in the structure of the lungs and the 

 presence of air-sacs, that it must be considered in some detail. 1 



LUNGS AND AIR-SACS OF BIRDS. 



When the ventral body-walls of a Bird are removed, the heart, stomach, 

 liver, and intestines, are seen pressed towards the mid-line, and on either side 

 of them a tightly-stretched fascia, the oblique septum, is observable, which 

 shuts them off from a paired lateral sub-pulmonary chamber (Fig. 209, 

 D.th.a). Other chambers are situated in the anterior thoracic region, ventral 

 to the lungs, which latter lie close against the vertebral column and the heads 

 of the ribs, by which they are impressed : others, again, are seen in the region 

 of the heart and in the posterior part of the abdominal cavity. 



These chambers are occupied by the air-sacs, the development and 

 physiological function of which will be described later on. 



The most posterior chamber on either side encloses an abdominal 

 (posterior) air-sac (Fig. 209, r.Abd.S., l.Abd.S). In Apteryx, this is 

 completely closed in by the oblique septum, but in other Birds it gives rise to 

 a large diverticulum which extends between the corresponding kidney and the 

 body-walls, and even into the latter, as well as between the pelvic muscles. 

 Its volume is naturally dependent upon the state of distension of the viscera 

 at the time. 



In front of this there are two air-sacs lying above and externally to the 

 oblique septum, and constituting the main part of the sub-pulmonary chamber ; 

 these may be called the anterior and posterior intermediate sacs (Fig. 

 209, f, ft)- A-teaasverse dividing- wall (s 1 ) lies between these two, at the 

 level of the cceliac artery, and a second septum (*) shuts off the anterior 

 intermediate sac from the one lying in front of it, to be described presently. 

 The posterior intermediate air-sac presents the simplest and most constant 

 relations, and never communicates with any of the neighbouring chambers, as 

 is often the case with the anterior intermediate. 



A pair ofprebronchial air sacs lies on either side of the oesophagus 

 above the bronchus, anterior to the hilum of the lung (Fig. 209, (7, (7), and 

 below this a sub-bronchial sac is situated, which is separated behind from 

 the anterior intermediate sac by a septum (Fig. 209, *). This is usually 



1 For Figs. 209 and 210, as well as for many of the details in the above 

 description, we are indebted to Professor H. Strasser of Freiburg in Baden, who has 

 kindly allowed us to make use of the manuscript of a paper which is not yet 

 published. The terms used are those of Professor Huxley. 



s 2 



