ORGANS OF CROCODILIANS. 



639 



be closed by valves, and the eyes by transparent third eyelids, and 

 the ears by movable flaps, so that the head can be comfortably im- 

 mersed ; a flat tongue is fixed to the floor of the mouth, and the 

 cavity of the mouth is bounded behind by two soft transverse mem- 

 branes, which, meeting when the reptile is drowning its prey, pre- 

 vent water rushing down the gullet ; 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 un- 

 impeded. 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 ab- 

 dominal 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 passages 

 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 opening lies 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, ; arid a 

 right aortic arch containing arterialblood, 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 haemo- 

 globin and others oxyhoemoglobin. At the roots of the two aortic 

 arches there is a minute communication between them the foramen 

 Panizzse. 



42 



P: 



FIG. 352. Half of the pelvic 

 girdle of a young crocodile. 



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

 ischium ; P., pubis or epipubis. 



