AIR SACS. 447 



lungs of the two sides and is continued a little ventral to the lungs 

 by the opposed median walls of the thoracic air sacs (see above). 

 The outer edges of the pulmonary aponeurosis are inserted into the 

 thoracic wall at the edge of the lung, and the posterior part of 

 it contains muscles which arise from the vertebral parts of the 

 ribs. The dorsal walls of the interclavicular, anterior thoracic 

 air-sacs, and of a small part of the posterior thoracic sacs, and 

 possibly of a small part of the anterior region of the abdominal 

 air-sacs are adherent to this aponeurosis. The pulmonary apo- 

 neurosis has been compared to the mammalian diaphragm, but 

 it is very doubtful if this homology can be maintained, for it does 

 not cut off the whole of the lung from the body cavity, the air- 

 sacs extending through it, and the pericardium lies altogether 

 outside it. 



However this may be, there can be no question that the so-called 

 septum has nothing to do with a diaphragm with which it also has been 

 compared. Indeed the oblique septum which is described by authors as 

 dividing the body cavity into a dorsal pulmonary part and a ventral 

 visceral part, containing the pericardum and viscera, has no separate ex- 

 istence. It consists merely of those parts of the walls of the interclavicular 

 and thoracic air- sacs, and possibly also of the anterior end of the abdo- 

 minal, which are not adherent to the body wall, to the lung surface, or 

 to each other, i.e. it consists mainly of the ventral walls of these sacs, and 

 does not divide the body cavity at all, there being no portion of that cavity 

 dorsal to it. 



The abdominal sacs lead into the cavities of the femora and 

 pelvic bones, while the interclavicular sacs are prolonged into 

 the air-spaces of the bones of the arm. and may extend into the 

 sternum and into the pectoral muscle. The extensions of the 

 cervical sacs have already been referred to. In many birds the 

 extensions of the air-sacs are much greater than those described. 

 In some they send extensions beneath the skin. In some of the 

 large swimming birds the cutaneous prolongations are so numer- 

 ous that the skin emits a crackling sound when touched. The 

 pneumaticity of the bones has been already referred to. It is 

 greatest in the screamers and hornbills. in which all the bones 

 are pneumatic and an extensive system of subcutaneous air-sacs 

 exists. The air-cavities in the bones of the head are diverticula 

 of the nasal passages, tympanic cavity, etc. 



The mesobronchium and secondary bronchi (entobronchia) 

 give off tertiary bronchi (parabronchia) which sometimes anas- 



