108 A MANUAL OF VETERINARY PHYSIOLOGY 



The greatest amount of expansion occurs where there is the 

 greatest amount of spongy lung-tissue and the least amount of 

 bronchial ramification, and this necessarily exists in the outer 

 zone of the lungs, particularly towards that portion facing the 

 diaphragm, and corresponds to that part of the chest where the 

 greatest movement occurs. 



From this it must not be inferred that the lung in other places 

 does not expand. As a matter of fact, the lungs fill up in the 

 living animal every crevice not occupied by heart or vessel. 

 There is no such thing as a pleural cavity in the healthy chest. 

 The lungs lie close against the ribs, and never leave ribs or 

 diaphragm during the whole life of the animal so long as health 

 exists. There is, of course, a space between the costal and 

 pulmonary pleura, but in health it is only a potential space. 

 A finger introduced into the chest finds the lung close up against 

 the opening which has been made, and if an attempt be exercised 

 to inject fluid into the potential pleural sacs, the close applica- 

 tion of the lung against the chest wall prevents it as effectually 

 as it prevents a piece of soft rubber tubing from being passed 

 along behind the costal and pulmonary surfaces. The lung 

 exposed through this small opening may even bulge outwards 

 under the pressure of the inspiratory act. 



These facts must not be confused with those which may be 

 observed under pathological conditions, when, the lungs having 

 collapsed under hydrostatic pressure, as in the case of effusion 

 into the chest, air readily enters the cavity through the cannula 

 when the level of the fluid is lowered by tapping ; for the lung 

 is no longer in contact with the chest wall, having collapsed 

 upwards towards the spine. 



The areas affected in pneumonia might indicate which are the 

 essential and which the non-essential portions of the lung in 

 ordinary respiration. Pneumonia always affects the lower lobes 

 of the lung, and nearly always the anterior lobes as well. The 

 latter, owing to the curve of the spine, are on a lower plane than 

 the upper lobes. It might be supposed from this that the upper 

 part of the lung was of more respiratory importance than the 

 lower, for with this surface of lung left the pneumonic patient 

 can live for days. But in the upright animal the lower part of 

 the lung is also the first in which pneumonia appears, and the 

 lower part in the biped is the extreme upper part in the quad- 

 ruped, from which it would appear that it is not necessarily the 

 least efficient portion of the lungs in which pneumonia begins, 

 though doubtless in the horse the least useful portion of both 

 lungs is the anterior lobes. 



The fact, however, remains that when air enters the lungs 

 some parts expand much more than others. They do not behave 



