382 



COMPARATIVE ANATOMY 



7' 



genetically a process of centrifugal 

 budding of the endodermic epi- 

 thelium followed by a further 

 development of the mesodermic 

 portions gradually results in a 

 complicated structure of the lung 

 such as occurs in the higher forms. 



Birds. The respiratory ap- 

 paratus of Birds presents so many 

 remarkable peculiarities, both as 

 regards the structure of the lungs 

 and in the presence of air-sacs, that 

 it must be considered in some 

 detail. Physiologically, it reaches 

 the highest degree of perfection 

 amongst Vertebrates. 



The comparatively small but 

 highly vascular lungs (Figs. 290 

 and 291) are closely applied to the 

 thoracic vert ^brae and heads of the 

 ribs, and are but slightly elastic 

 and capable of very little disten- 

 sion. The lower surface of each 

 lung is closely invested by a thin 

 fibrous membrane, the pulmonary 



A, , f B r n ii aponeurosis, 1 into which are in- 



g a I / / r ' f m serted a variable number of 



muscular bands (eosto- pulmonary 

 muscles) : these arise from the 

 vertebral ribs, and are supplied 

 by the intercostal nerves. 



The main bronchus (mesobron- 

 chium) enters the lung at about 

 the middle of its ventral surface, 

 and gradually losing its carti- 

 laginous rings, extends to the 

 posterior end, giving off secondary 

 bronchi. Of these, eight are ven- 

 tral (entobronchia) and six to ten 

 dorsal (ectobronchia), and the 

 branches of small calibre which 

 arise v from them are perforated 

 by numerous close- set apertures 



leading into small tubes the so-called " lung-pipes " (para- 

 bronchia). These again, give off short,. radially arranged bronchioli 



1 The pulmonary aponeurosis, as well as the oblique septum, is often spoken 

 of as a "diaphragm" (cf. p. 184). 



FIG. 289. LUNGS or Chamcdco 

 monachus. 



T, trachea. 



