72 



RESPIRATORY MECHANISMS 



of about 10 mikrons diameter. The parabronchi anastomose 

 freely, as do also the air-capillaries from different parabronchi. 

 The walls of the parabronchi are covered, like those of the 

 larger bronchi, with a ciliated epithelium and contain numer- 



Fig. 44. Lungs of duck with ostia, opening into air-sacs. (Sappey.) 



ous smooth muscle fibres. The walls of the alveoli appear to 

 be made up of the endothelium of blood capillaries only, with- 

 out any epithelium. It is a very significant fact, discovered by 

 Schulze (1910) and by Juillet (1911), that "recurrent" or 

 "saccobronchi" penetrate the lung from the posterior air-sacs 

 (Fig. 45 s.b. and Fig. 42). These recurrent bronchi are 



