370 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 50 



It crosses the ventral surface of the lung diagonally, whereby it is 

 enabled to assist in the expansion of the pulmonary cavity. The 

 upper muscle of the last rib also follows a somewhat oblique course. 

 All these muscles are innervated by branches of the intercostal 

 nerves. 



The Abdominal Diaphragm (Fig. i, DIA) 



(Posterior, oblique, vertical or abdominal diaphragm; Oblique septum, Hux- 

 ley; Diaphragme thoraco-abdominal, Sappey; Diaphragme posterieur, 

 Milne Edwards.) 



This membrane, which is likewise aponeurotic, appears to be a 

 continuation of the ventral border of the mediastinum. It extends 

 from that border on either side, obliquely to the wall of the body 

 cavity, and is attached in front to the pericardium. It therefore 

 incloses a wedge-shaped space (the cavum cardio-abdominale of 

 Huxley), whose base is at the sternum and whose sharp edge is at 

 the ventral border of the mediastinum. It reaches forward as far 

 as the pericardium, and appears there to be shut off by the heart. 

 Backwards, it extends as far as the posterior extremity of the ab- 

 dominal cavity. In it lie the heart and all the other viscera. 



Symmetrically placed on either side of this wedge-shaped space there 

 is a space similar in form whose sharp edge is formed by the union of 

 the pulmonary diaphragm with the outer surface of the abdominal 

 diaphragm. The bases of these secondary cuneiform spaces, the cava 

 subpulmonalia of Huxley, are the lateral walls of the body cavity. 

 Each one of these two spaces is divided by four septa into as many 

 portions, each of which is occupied by an air-sac. The first sac lies, 

 as already mentioned, in the cavum pulmonale. The second extends 

 towards its symmetrical companion of the opposite side and unites 

 with it, where the cavum cardio-abdominale, that elsewhere separates 

 these two cavities, is wanting. The arrangement of the air-sacs is 

 dependent on the development of the diaphragms, which are never 

 pierced by air ducts. When pneumatic connections are present they 

 are in every case extra-thoracic. The general plan of the system is 

 as follows : 



In the cavum cardio-abdominale lies the saccus cervicalis ; 



In the cavum subpulmonale, loculus primus, lies the saccus inter- 

 clavicularis ; 



In the cavum subpulmonale, loculus secundus, lies the saccus inter- 

 medins anterior ; 



In the cavum subpulmonale, loculus tertius, lies the saccus inter- 

 medins posterior; 



