ORGANS OF CROCODILIANS. 



617 



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 un- 

 impeded. P'or 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 "croco- 

 dile's tears." 



The ears open by horizontal slits, 

 over which lies a flap of skin ; 

 three Eustachian tubes one me- 

 dian and one on each side open 

 into the mouth behind the pos- 

 terior nares. 



The nostrils also can be closed, 

 and, as we have already noticed, 

 their internal opening lies at the 

 back of the mouth. 



The stomach suggests a bird's 

 gizzard, for it has strong muscular 

 walls, and itspyloricend 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 arch 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 hemoglobin and others oxyhiemo- 

 globin. At the roots of the two aortic arches there is a minute com- 

 munication between them the foramen Panizzx. 



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

 venae 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 cceliac 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- 



FIG. 305. Half of the pelvic girdle 

 of a young crocodile. 



//., Ilium; a.f., acetabulum ; /s., 

 ischium ; /'., pubis or epipubis. 



