H2 A MANUAL OF VETERINARY PHYSIOLOGY 



sternum is fixed to the ribs and undergoes little or no move- 

 ment ; even the most powerful respiratory movements in the 

 horse give rise to no sternal movement. On the other hand, there 

 is a moderate amount of movement between the last two or 

 three sternal ribs and the cartilages. During laboured respira- 

 tion any muscle which can assist in advancing the ribs directly 

 or indirectly is brought into play. This is well marked in 

 dyspnoea. 



After the expiratory act there is a pause before the next 

 inspiration. In the horse at rest the period of expiration is as 

 a rule longer than that of inspiration, though the proportion 

 between the two is not invariable. During work the duration 

 of the inspiratory and expiratory acts is about equal. 



Intrapulmonic and Intrathoracic Pressure. — During inspiration 

 a slight negative pressure exists in the trachea, and during 

 expiration a slight positive pressure. A strong inspiration, 

 with the glottis closed, such as can be made by a voluntary 

 effort in man, rarefies the air in the respiratory passage and the 

 pressure falls. Conversely a strong expiratory effort, with the 

 glottis closed, as in coughing, defalcation, or parturition, raises 

 the pressure in the respiratory passages considerably, and thereby 

 increases the intrathoracic pressure. The increased pressure 

 within the respiratory passages and thorax influences blood- 

 pressure, for the veins leading to the heart are partly obstructed, 

 as may be witnessed, for example, in man in a violent fit of 

 coughing, and thereby the venous flow to the heart is lessened. 

 In the pleural cavity a negative pressure is always present, due 

 to the tendency of the elastic lungs to collapse. The value of this 

 pull of the lungs on the chest wall has been ascertained for the 

 sheep to be about 3 mm. (J inch) of mercury, and during dyspnoea 

 9 mm. (f inch). In the dog during inspiration the negative 

 pressure in the pleural sac is 6 mm. (J inch) of mercury, whilst 

 during expiration 3 mm. (J inch), has been observed. In the 

 horse 6 mm. (J inch) has been registered during a powerful 

 expiration, and 28 mm. (ij inches) during a powerful inspira- 

 tion. The negative pressure can be recognised post mortem by 

 the rush of air immediately the chest is punctured. 



When the chest is opened during life, the atmospheric pressure 

 within and without the lungs is equal, and in consequence the 

 lungs shrink and in time collapse. In the horse a wound of 

 the chest cavity is serious, for the reason that the two pleural 

 sacs communicate, and therefore both lungs shrink and finally 

 collapse. The collapse is not immediate, but gradual; the 

 shrinkage is immediate. The collapse leads to death by asphyxia, 

 unless the chest wound can be perfectly sealed, in which case 

 the air in the chest cavity is absorbed, and the negative thoracic 



