THE RESPIRATORY SYSTEM. 405 



vagus is more efficient than in other races of dogs and in rab- 

 bits kept in coniinement; and possibly this may in part account 

 for the greater speed and especially the endurance of the 

 former. The very conformation of some animals, as the grey- 

 hound, with his deep chest and capacious lungs, indicates an 

 unusual respiratory capacity. 



The law of habit is isjpll illustrated in the case of divers, who 

 can bear deprivation of air longer than those unaccustomed to 

 such submersion in water. Greater toleration on the part of 

 the respiratory center has probably much to do with the case, 

 though doubtless many other departures from the normal occur, 

 either independently or correlated to the changes in the respira- 

 tory center. Some mammals, like the whale, can long remain 

 under water. 



Summary of the Physiology of Eespiration.— The purpose 

 of respiration in all animals is to furnish oxygen for the tissues 

 and remove the carbonic anhydride they produce, which in all 

 vertebrates is accomplished by the exposure of the blood in 

 capillaries to the atmospheric air, either free or dissolved in 

 water. A membrane lined with cells always intervenes between 

 the capillaries and the air. 



The air may be pumped in and out, or sucked in and forced 

 out. 



Respiration in the Mammal— The air enters the lungs, 

 owing to the enlargement of the chest in three directions by the 

 action of certain muscles. It leaves the lungs because of then- 

 own elastic recoil and that of the chest- wall chiefly. Inspiration 

 is active, expiration chiefly passive. 



The diaphragm is the principal muscle of respiration: In 

 some animals there is a well-marked facial and laryngeal as 

 well as thoracic respiration. Eespiration is rhythmical, con- 

 sisting of inspiration, succeeded without appreciable pause by 

 expiration, the latter being in health of only slightly longer 

 duration. There is also a definite relation between the number 

 of respirations and of heart-beats. According as respiration is 

 normal, hurried, labored, or interrupted, we describe it as 

 eupnce, hyperpnoea, dyspncea, and apncea. The intra-thoracic 

 pressure is never equal to the atmospheric — i. e., it is always 

 negative — except in forced expiration ; and the lungs are never 

 collapsed so long as the chest is unopened. The expired air 

 differs from that inspired in being of the temperature of the 

 body, saturated with moisture, and containing about 4 to 5 per 



