862 SPECIAL PHYSIOLOGY. 



THE ORGANS AND FUNCTION OF RESPIRATION IN ANIMALS. 



As already stated, respiration is usually either aerial, or aquatic, according 

 to the medium in which an animal is fitted to live ; but, in a few cases, both 

 kinds of respiration are possible, as in the true Amphibia. Examples of aerial, 

 and of aquatic breathers, are met with in the Vertebrate, Molluscous, and 

 Articulate Subkingdoms ; but in the Molluscoida and Annuloida, as well as 

 in the Coelenterata and Protozoa, the respiration is, in all cases, purely aquatic. 

 The respiratory organs afford no grounds for classification. 



Aerial Respiration. 



The general principles, physical and chemical, on which this kind of respi- 

 ration is performed in animals, are the same as those which govern the respi- 

 ratory process in Man ; but the organs concerned vary according to the animal, 

 and exhibit wide departures from the form and structure of the apparatus in 

 Man, as we descend in the scale. 



Vertebrata. In all Mammalia, the respiratory apparatus is similar in plan, 

 and even in detail, to that of the human body. There is a complete thorax 

 with movable walls, separated from the abdominal cavity by a perfect dia- 

 phragm, and containing lungs suspended freely in pleural chambers, resem- 

 bling, in all essential particulars, those of Man. The respiratory movements 

 are performed in the same manner ; their frequency also has a general relation 

 to that of the pulse. The respirations are fewer, like the beats of the heart, in 

 the larger Mammalia than in the smaller ones, these latter requiring relatively, 

 more frequent changes of air in the lungs, to maintain sufficient respiratory 

 action for the development of heat, and for other purposes in their economy. 

 In the Carnivora, the lungs are relatively much larger than in the Herbivora. 

 The right lung is usually the larger. In the horse, and elephant, and in most 

 Cetacea, they are simple in form ; but more commonly they are divided into 

 lobes ; usually on the left side, these do not exceed three, and, on the right, 

 five lobes. 



In Birds, besides a typical symmetrical arrangement, as to position and size, 

 important peculiarities in the respiratory apparatus are met with. The thorax 

 and abdomen- form but a single cavity, there being usually a rudimentary dia- 

 phragm only, which is spread out upon the base of the lungs, as in some 

 Reptiles. In the ostrich tribe, however, the diaphragm approaches, by its 

 greater development, the Mammalia character, and in the Apteryx, this 

 musculo-tendinous partition is quite perfect. The thoracic walls are constructed 

 on a modified plan. The sternum, here expanded into the large breastbone, 

 which gives attachment to the muscles of flight, forms the greater part of these 

 walls, and even supports the abdominal viscera ; whilst the ribs, which have 

 a peculiar angular joint between their sternal and vertebral portions, occupy 

 proportionally a smaller part, The absence of the diaphragm, and the diffi- 

 culty of expanding a thorax thus constructed, by any active inspiratory move- 

 ment, have led, as it were, to a complete reversal of the mechanism by which 

 the air is drawn into, and expelled from, the chest. In Mammalia, and in 

 Man, inspiration is an active, whilst expiration is, to a large extent, a passive 

 movement ; but in Birds, expiration is active, whilst inspiration is chiefly, if 

 not entirely, passive. In expiration, the large sternum is drawn towards the 

 vertebral column by muscular effort ; the ribs are approximated,, and bent at 

 the above-mentioned angles, and so the thoracic part of the thoracico-ab- 

 dominal cavity, and therefore its contained lungs, are compressed, and air is 

 driven from them. The expiratory muscles now cease to act, and the sternum, 

 chiefly by the elastic resilience of the bent ribs, being drawn from the vertebral 

 column, pressure is removed from the surface of the lungs, and air is inspired ; 

 the expansion of the lungs is probably favored by the contraction of the in- 

 complete diaphragm, which is attached to their base. The condition of the 

 thorax and lungs when at rest, corresponds with the state of distension, whilst 

 active breathing begins by an effort to force air from the chest, and not to draw 



