T38 A MANUAL OF VETERINARY PHYSIOLOGY 



Cause of First Inspiration. — The cause of the first act of 

 inspiration in the foetus is that the placental circulation being 

 cut off, the respiratory centre of the foetus becomes stimulated 

 through the increased venous character of the blood now cir- 

 culating through it ; as a result of this, inspiration is automati- 

 cally produced. But it is also assisted by reflex impulses carried 

 from the surface of the skin due to handling and drying ; handling 

 the skin of the foetus while still in utero with the placental cir- 

 culation intact may provoke respirations, and in all animals the 

 very first act of the mother is to dry the foetus and stimulate 

 the skin by licking. 



The Amount of Air Required. — Numerous respiration experi- 

 ments have been made on all animals to determine the amount 

 of air they require and the gases of respiration. The horse is, 

 of all others, the one to which perhaps the greatest practical 

 interest attaches in this respect, though a knowledge of it in 

 connection with other animals is of value. 



The lungs of a horse will contain about 42*5 litres (1 \ cubic feet) 

 of air at the end of a deep inspiration. During ordinary repose 

 he draws into them 2,265 to 2,548 litres (between 80 and 90 cubic 

 feet) of air in the hour, though considerable variation may be 

 found even in the same animal. 



An average inspiration in the horse during repose amounts 

 to about 41 litres (250 cubic inches), and the amount of air 

 which flows in and out during ordinary quiet respiration is 

 known as the tidal air. Speaking roughly, it is only one-tenth 

 of what the lungs can contain ; the remaining nine-tenths are 

 made up of complemental, reserve, and residual air. The com- 

 plemental air is that over and above the tidal which can be 

 taken in by a forced inspiration, while the reserve is a some- 

 what similar amount which can be expelled by a forced expira- 

 tion. The most powerful expiratory effort is unable to remove 

 from the lungs all the air they contain, and this amount is known 

 as the residual air. 



The great variations which have been observed in the amount 

 of air taken in by the same animals, under apparently identical 

 conditions, cannot be adequately explained ; the slightest dis- 

 turbing influence alters both the rhythm and depth of the 

 respirations. Under the influence of work, the amount of air 

 required is greater, and as a rule, the faster the pace the more 

 air needed ; but many disturbing factors occur which render 

 experiments on this subject very contradictory, and productive 

 of the greatest variation. During severe work, such as a gallop, 

 a horse is taking air into his lungs to the extent of 24,067 litres 

 (850 cubic feet) per hour at least, and probably more ; the 

 respirations, from being 9 to 10 per minute during repose, may 



