CHARACTERISTICS OF CROCODILIANS. 485 



by two soft transverse membranes which, meeting when the reptile 

 is drowning its prey, prevent water rushing down the guUet ; the 

 posterior opening of the nostrils is situated at the very back of the 

 mouth, and when the booty is being drowned, the Crocodilian keeps 

 the tip of its snout above water, the glottis is pushed forward to meet 

 the posterior nares, a. complete channel for the passage of air is thus 

 established, and respiration can go on unimpeded. For their shore 

 work the Crocodilians prefer the darkness, but they often float basking 

 in the sun, with only the tip of the snout and the ridge of the back 

 exposed. 



Glands with a secretion which smells like musk are usually developed 

 on the margin of the lower jaw, at the side of the cloacal aperture, and 

 on the posterior margins of the dorsal scutes. The musky odour is very 

 strong during the pairing season, and when the animals are attacked. 



In connection with the muscular system, the presence of what is often 

 called an incipient diaphragm between the thoracic and the abdominal 

 cavity is of interest. 



The brain seems very small in relation to the size of the skull. 

 The eyes are provided with a third eyelid, as in most Reptiles, Birds, 

 and Mammals ; there are large lachrymal glands, but there is no special 

 deceitfulness about " crocodile's tears." 



The ears open by horizontal slits, over which lies a flap of skin ; three 

 Eustachian tubes — one median and one on each side— ^open into the 

 mouth behind the posterior nares. 



The nostrils also can be closed, and, as we have already noticed, their 

 internal openings lie at the back of the mouth. 



The stomach suggests a bird's gizzard, for it has strong muscular walls, 

 and its pyloric end is twisted upward so as to lie near the cardiac part. 



The heart is four-chambered, the septum between the ventricles being 

 complete as in Birds and Mammals. But as the dorsal aorta is formed 

 from the union of a left aortic arch containing venous blood, and a 

 right aortic containing arterial blood, the blood which is driven to 

 many parts of the body is "mixed blood," i.e., blood partly venous, 

 partly arterial, with some of its red blood-corpuscles carrying hsemo- 

 globin and others oxy-hjemoglobin. At the roots of the two aortic 

 arches there is a minute communication between them — the foramen 

 Panizzas. 



Into the right auricle venous blood is brought by the two superior 

 venre cavae and by the inferior vena cava. The blood passes through 

 a valved aperture into the right ventricle, and is driven thence (a) by 

 the pulmonary artery to either lung, or {b) by the left aortic arch to the 

 body. From this left aortic arch, before it unites with its fellow on the 

 right to form the dorsal aorta, is given off the great cosliac artery. The 

 anterior viscera thus receive wholly venous blood from the heart. 



The blood driven to the lungs is purified there, and returns by pul- 

 monary veins to the left auricle. Thence it passes through a valved 

 aperture into the left ventricle. Thence it is driven into the right aortic 

 arch. From this the carotids to the head and the subclavians to the 

 fore-limbs are given off. These parts of the body thus receive wholly 

 arterial blood from the heart. 

 The venous blood returning from the posterior regions may pass 



