Respiration. 79 



The increase in the size of the chest which occurs during 

 quiet inspiration in the horse is stated by Colin to be as 

 follows: the antero-posterior or longitudinal diameter of 

 the chest is lengthened by 4 to 5 inches, and the transverse 

 diameter between the eleventh and twelfth ribs increased 

 by H inches. 



Only the last ten pair of ribs take, under ordinary cir- 

 cumstances, any share in respiration ; this is due to eight 

 true ribs being covered by the scapula ; when, however, a 

 difficulty occurs in the breathing, the elbows are turned 

 out which brings other muscles into play as auxiliaries 

 in respiration, and a certain number of the true ribs now 

 assist in increasing the capacity of the chest. 



The Movements of the Diaphragm during respiration are, 

 as might be imagined from the attachments of the muscle, 

 of a peculiar kind. The diaphragm works to and fro, not 

 equally over its whole surface, for the central portion 

 moves but very little owing to its connection with the pos- 

 terior vena cava, and the part of the diaphragm below this 

 vessel is so short that its movement is very limited. 



The chief motion in the diaphragm lies in its upper part. 

 At each inspiration this recedes, carrying back with it the 

 liver, stomach, and spleen. At each expiration it advances, 

 carrying with it the viscera which were previously dis- 

 placed. 



In Fig. 4 (after Sussdorf) the body is supposed to be 

 divided horizontally. The position of the diaphragm and 

 the displacement of the viscera during respiration are 

 clearly seen. 



Expiration. — The chest having been filled with air, the 

 next process is its expulsion, and the mechanism here con- 

 cerned is not fully agreed upon by physiologists. Whilst some 

 hold that it is a purely non-muscular act, others contend 

 that certain muscles do share in the process. All are 

 agreed that the elastic reaction of the lung induces it to 

 retract, the effect being to draw the diaphragm forward, 

 moreover, that the elastic recoil of the cartilages of the 

 false ribs decreases the diameter of the chest and assists 



