396 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 50 



thoracic air-spaces. This certainly was not justifiable, for, taking 

 them altogether, especially if we include among them the non- 

 dilatable air-spaces in the pneumatic bones, their volume is by no 

 means insignificant. That the ventral wall of the' body appears some- 

 what drawn inward during inspiration does not prove that there is 

 a decrease of volume during this respiratory phase, but it shows, at 

 least, that there can be no noticeable enlargement of the abdominal 

 sacs during inspiration. In the sacci intermedii the increase of 

 volume during inspiration is also in realty not so great as at first 

 sight appears. 



There is, of course, an increase in the size of the thoracic space 

 during inspiration. This is effected by a movement of the sternum 

 downward and forward, whereby the angles between the vertebral 

 and sternal pieces of the ribs are increased. Thus the thoracic 

 space gains in height. The increase of transverse dimensions that 

 occurs at the same time is insignificant. Dorsally the lungs adhere 

 to the ribs, and even extend between them. To the concave ventral 

 surface of the lungs are attached the muscle of Perrault which con- 

 tract during inspiration. By the enlargement of the thoracic cavity 

 the atmospheric pressure in the thoracic air-sacs is reduced. These 

 air-sacs are chiefly situated ventrally from the lungs. The differ- 

 ence between the (lower) pressure in these sacs and the (higher) 

 pressure outside will indeed be diminished in amount during inspira- 

 tion by the influx of air through the ostia into the sacs ; yet on 

 account of the smallness of the ostia some time must always elapse 

 before the pressure within and without is completely equalized. 

 Until this equalization is complete the air-sacs below the lungs, in 

 which the pressure is low, and the muscles of Perrault must draw 

 the ventral surface of the lungs downwards, thus effecting an expan- 

 sion of the lungs. Baer himself has (1896, p. 476) pointed out that, 

 in the first period of inspiration, there is a relatively very low air- 

 pressure in the sacci intermedii. The consequence of this is that at 

 that respiratory phase the ventral wall of the abdomen sinks in, not- 

 withstanding the fact that it is very thick and muscular, and that it 

 tends to become tense during inspiration and opposes a marked resist- 

 ance to the depression. The thin wall of the pulmonary diaphragm 

 certainly cannot withstand such a difference of pressure better than 

 this external body wall ; it must be pulled downward, and when the 

 pulmonary diaphragm is thus depressed the lungs must necessarily 

 expand. That no complete equalization occurs between the external 

 and internal air pressure during inspiration is stated by Baer (1896, 

 p. 478) in the following words: "Bevor noch der Spannungsunter- 



