106 A MANUAL OF VETERINARY PHYSIOLOGY 



reduction in the amount of lung inflated. Fig. 46, taken from 

 Ellenberger and Baum, slightly modified, shows the position of 

 the lungs during normal inspiration.* The lungs are resting on 

 the diaphragm, and it will be observed that this muscle may be 

 seen passing to the ribs for attachment. During ordinary inspira- 

 tion the portion of the thoracic cavity not occupied by the lungs 

 is temporarily obliterated by the abdominal viscera being thrust 

 forward, and thus pressing the diaphragm against the ribs and 

 so closing the potential space. In Fig. 46, if the lungs were 

 fully distended during the forced inspiration of work, they would 

 extend to where the diaphragm is attached to the ribs, viz., 

 about four inches above the edge of the false ribs. In Fig. 44, A, 

 the dotted line under the loin indicates the limiting position of 

 the lungs during a forced inspiration. During the ordinary in- 

 spiration of rest the dotted line would end further forward. The 

 pressure behind the diaphragm can be realised by an inspection 

 of Ellenberger's figures (Figs. 45 and 46). With this pressure 

 against the diaphragm, especially during fast or heavy work, 

 there is no difficulty in understanding why this muscle so fre- 

 quently ruptures in the horse, and, as we shall see later on, it 

 may help to explain the far more common lesion of ruptured 

 stomach. During expiration Ellenberger represents the lungs as 

 occupying a relatively very small portion of the thoracic cavity. 

 His figure is not reproduced, as we think the space allotted to 

 the collapsed lung is likely to prove misleading. 



The Lungs Fill with Air. — Air is never absent from the healthy 

 lungs, even after death. On life ceasing they retract, especially 

 from the diaphragmatic region, and shrink upwards towards the 

 spine. If they be now distended artificially, the organs gradually 

 fill, those portions nearest the bronchus distending first. There 

 is no sudden or unusual swelling, but rather a gradual invasion 

 of different but neighbouring areas, which gradually come nearer 

 and nearer together, and finally meet. The parts to expand last 

 are the edges of the lungs and the surface in contact with the 

 diaphragm, especially with its upper half. It is not intended to 

 infer from these post-mortem observations that the distension 

 of the lungs during life is identical, because during life they are 

 never collapsed to the same degree, and, further, when distended 

 after death, they inflate in the line of least resistance. Never- 

 theless, all parts of the lung are not equally distensible. The. 

 part situated around the roots of the lungs must necessarily, 

 from the obstruction offered by the bronchi, be less distensible 

 than those parts further removed. The anterior lobes, in con- 

 sequence of the rigid nature of this part of the thoracic cage, 

 can hardly move at all, though, of course, they are kept distended. 



* The writer's view is that the lung is not shown sufficiently distended. 



