248 NATURAL HISTORY. 



positively within the chest. Watch the land or water Chelonians, and they seem never to 

 breathe ; it is only when these last have been under water for some time that bubbles come up 

 from their mouths. There is no vigorous chest movement resembling respiration. In fact, the 

 Cheloniaii lungs have air forced into them by a process not unlike that of swallowing. Keep a 

 Tortoise's mouth open too long, and it will be suffocated for want of air, or close the nostrils, leaving 

 the mouth free, and the same result will follow. The Chelonians breathe by keeping the mouth firmly 

 closed, and by the action of certain muscles in the throat, some of which pull down the hyoid bone 

 and the bone of the tongue, and others which restore it to the usual position. The hyoid bone being 

 dragged down, air rushes in at the nostril, gets into the mouth, and then the tongue closes the internal 

 nostril, whose position is at the back of the palate. Then, as the bone and tongue are moved upwards 

 again, the air is forced down into the lungs, and the air-cells are filled. The expiration of the air is 

 produced by the collapsing of the air-cells, and it passes slowly upwards through the main air-tube to 

 the mouth. The breathing is very slow. In the Chelonians, and all Reptilia, the midriff muscle is 

 absent, the chest and abdominal cavities being continuous, and all the viscera are covered with a 

 membrane, called pleuro-peritoneal. 



The lungs of the Chelonia are large and occupy much space ; in some, as in the Alligator Terrapin, 

 they are divided into several compartments, and the air-cells and the 

 breathing-surface generally appear to be the most complicated near the 

 entrance of the main air-tube. Like those of all Reptiles, the lungs of 

 the Chelonians contain cold blood, and the chemical and physiological 

 changes in it are incomplete and slow. No brilliant red stream is 

 passed from the lungs to the heart in the Reptiles, for the blood therein 

 flowing is dark a mixture of oxygenised and imperfectly aerated 

 blood and it is sent forth in that state by the ventricle of the heart all 

 over the body and into the lungs. The auricles of the heart, which 

 are very capacious in the Chelonia, are situated above and before 

 the ventricle, and are divided by a partition. The ventricle for there 

 is only one is muscular, and the blood rushes from the two auricles 

 into it and gradually distends it. In the Common Tortoise the ventricle 

 is little more than a simple cavity, but in the Hawk's-bill Turtle the 

 cavity is divided into several communicating compartments by mus- 

 cular projections and fibres, which strengthen the whole. But in both 

 SECTION OF LUNGS OF TORTOISE, and ^11 otlier instances the ventricle, by its contraction, expels the 



SHOWING CELLULAll AKKANGE- . . * 



MENT blood, not into the auricles again, but through a series of blood- 



vessels, namely, two distinct aortas, a right and a left, and a main 



vessel for the lungs or pulmonary artery. The blood of the great veins is thus mixed in the 

 ventricle with that from the lungs, and the mixture of oxygenated and non-aerated blood is sent forth, 

 some of it to the body, and some to the lungs. The two aortse pass backwards and unite opposite 

 the fifth dorsal vertebra, and thence but one vessel is continued to supply the body. 



Besides the system of veins and arteries, the Chelonians, as well as all Reptiles, possess large 

 sets of lymphatics, and they open into the great veins of the neck on each side. 



The blood corpuscles of the Chelonia, and of all Reptiles, are elliptical in shape, and are 

 larger than those of the Birds and Mammalia ; probably those of the Tortoise are T i^- of a line long 

 and T i T of a line broad. And experiment has shown that the temperature of the blood and of the 

 animal is very slightly above that of the air or water in which it may be placed for a while. 



Although some Chelonians can last without food for many months, others feed constantly ; and 

 whilst some enjoy vegetable substances others are carnivorous. Hence there is some diversity in the 

 structure of the digestive organs ; but it is only necessary to state that the flesh- and insect-eating kinds 

 have a shorter intestine than the others. In all there is a capacious gullet and oesophagus, but there 

 is no crop. The muscular coat of this passage is strong, and in the Turtles the inner surface is lined 

 with long, hard projections pointing towards the stomach, denying return to anything which has gone 

 down. The stomach is long, thick, cylindrical, and bent ; its walls are very muscular, and it is closely 

 connected with the liver, that of an American fresh- water kind (Emys concinna) being imbedded in it. 



