HISTORY AND HABITS OF THE TESTUDINATA. 
XVII 
The structure of the lungs and the condition of the respiratory function 
are strictly analogous to this imperfect state of the circulation. Instead of 
being formed, as in the warm-blooded animals, of a close congeries of innu¬ 
merable minute cells, far more compact than the most solid sponge, the cells 
are here large and comparatively few, and occupy, in some parts, the parietes 
only of the spacious sacs of which these organs consist. The quantity of blood 
exposed to the influence of the air at any one time, must therefore be very 
small, and the decarbonization in a corresponding degree imperfect. In exact 
correspondence with these facts, the mode of respiration is also extremely 
circumscribed. Deprived, by the osseous agglutination of the ribs and sternum, 
of all motion of the chest, and being also unprovided with a diaphragm, the 
inhalation of air is obviously impossible: inspiration is therefore performed 
by a simple act of deglutition. By the expansion of the pharynx, air is received 
into that cavity through the nostrils, the posterior openings of which are then 
closed by the application of the tongue, whilst the opening of the pharynx into 
the oesophagus is also shut, by the circular contraction of that part. The air 
tube is consequently the only passage left open, and by the contraction of the 
pharynx, , the air is forced through it into the lungs. Expiration is effected by 
means of certain muscles, hereafter to be described, which are situated at the 
back part of the body, and which, by their contraction, force the viscera of the 
abdomen forwards, and thus pressing upon the lungs, occasion the partial ex¬ 
pulsion of the air. Thus the structure of the lungs, the circumscribed mode 
of inspiration, and the partial evacuation of the lungs in expiration, all com¬ 
bine to render the respirato^ function imperfect; whilst the mixture of the 
two kinds of blood in the heart, also renders the more frequent or complete 
act of respiration unnecessary, as there always remains some aerated blood to 
mix with that which is impure. 
But the character by which these animals are at once distinguished from all 
other Vertebrata, is the solid bony box, within which all the viscera are inclosed, 
and which, in most genera, covers also the head, tail and limbs, when the 
animal is at rest. The upper portion of this osseous case, called the carapace 
or buckler, is formed of the dorsal vertebrae and ribs, which are immoveably 
connected by sutures, not unlike those of the cranial bones in most Mammifera, 
