340 



CLASSIFICATION OF VERTEBRATES. 



nerves are distinct in origin. A characteristic is the double 

 condition of the sympathetic in the region of the neck, one 

 portion following the vertebrarterial canal, the other the carotids. 

 The nostrils, except in Apteryx, are near the orbits. Con- 

 nected with the olfactory organ is a nasal gland, usually situated 

 in the frontal bone, its duct emptying into the respiratory 

 chamber. The eyes are large and highly developed, and are 

 spherical behind, obtusely conical in front. Except in Apteryx 

 there is developed a peculiar fold, the pecten or marsupium, which 

 is vascular, and projects into the posterior chamber in the line 

 of the choroid fissure. The nictitating mem- 

 brane is large, transparent, and is moved by 

 two special muscles, the quadratus and the 

 pyramidalis. The associated Harderian gland 

 is large, the lachrymal, at the external angle 

 of the orbit, being small. The eye muscles 

 proper are small ; and the eyeball is some- 

 what limited in its motions, this being com- 

 pensated by the flexibility of the neck. The 

 ear has a large semicircular canal ; and the 

 lagena is long and slightly coiled, its distal 

 end being somewhat expanded. It recalls 

 the cochlea of the mammals, but a Corti's 

 organ is lacking. The tympanic cavity sends 

 prolongations into the surrounding bones, 

 one, the siphoneum, penetrating the lower 

 jaw. The tympanum is crossed by the slender columella, which 

 bears at its membranal end a discoid stapedial plate. The 

 external ear is surrounded by a circle of feathers ; and in some 

 birds (e.g., owls) these may be moved, like a valve, by appro- 

 priate muscles, 



The heart is completely divided into right and left halves; 

 and the dorsal aorta is supplied only by the right aortic arch, 

 the left of the normal pair being converted into the innominate 

 artery. There is no mixture of arterial and venous blood in the 

 heart. The blood returns to the heart usually by two precavae 

 and a single postcava, these emptying separately into the right 

 auricle in which the sinus has become merged. There is no 



FIG. 335. Brain 

 of bird. 



