534 Comparative Morphology of Chordates 



change is augmented by a corresponding change in the convexity of the 

 cornea. Both structures become more convex for near vision, less so for 

 distant vision. These changes are effected by the system of small 

 ciliary muscles developed in the choroid layer and forming a muscu- 

 lar zone encircling the eyeball at the region of the periphery of the 

 cornea (Fig. 415, CM). 



The ear is well developed in all birds. In general structure it 

 closely resembles the reptilian ear at its best — namely, as it is in the 

 crocodilians. The tympanic mechanism is uniformly well developed. 

 The tympanic membrane is depressed below the surface of the skin, 

 lying at the bottom of a fairly deep external auditory meatus. 



The most significant difference between the ears of reptile and bird 

 is in the cochlea (see p. 208). It is longer in the bird (Fig. 180F) and 

 there is a definite twist in its axis suggestive of the spiral of the much 

 longer mammalian cochlea (Fig. 180G). With increase in the length of 

 the cochlear tube, its internal sensory organ of Corti is correspond- 

 ingly extended. Unquestionably this gives the bird, compared to the 

 reptile, a wider range of hearing and higher capacity for discriminating 

 qualities of sound. All of this is consistent with the fact that the birds' 

 highly specialized syringeal organs give them vocal abilities far sur- 

 passing the reptilian repertory of hisses, grunts, and the unmusical 

 but thunderous bellow of an old male alligator. The biologic utility of 

 the bird's song requires that those "birds of a feather" which, by 

 nature, "flock together" should have an ear adequate for hearing and 

 discriminating the songs. If, for man, the song is an element of beauty 

 in the world of life of which he is a part, it is only because it is his good 

 fortune to have an ear whose structure is closely similar to that of the 

 bird's ear. 



Brain. All of the major regions of the bird's brain are strongly 

 developed except the pallium. Viewing the brain dorsally or laterally 

 (Fig. 416), the conspicuous parts are the cerebral hemispheres, the 

 optic lobes, and the cerebellum. The hemispheres and cerebellum 

 are so enlarged that they nearly or quite meet dorsally and cover a large 

 part of the optic lobes which project laterally, while the diencephalon 

 is almost completely hidden. It is as if, in a brain shortened in the 

 manner described in discussing the skull, the expansion of hemispheres 

 and cerebellum had squeezed the optic lobes outward laterally. 



The relative proportions of the parts of the brain are always a good 

 index to the animal's sensory capacities and to the general nature of its 

 somatic (in distinction to visceral) motor activities. The larger the eye, 

 the greater is the number of sensory elements and, accordingly, the 



