184 
GENEBAL OMNITHOLOGY. 
owing to the relatively greater size and convexity of the cornea. The waters are enclosed in 
exceedingly delicate membranes j the vitreous in the hyaloid membrane, e, which, besides^ 
lining the posterior chamber and enclosing the lens as already said, sends thin partitions all 
through the vitreous humor to steady these glassy waters. 
The optic nerve, a, of birds is peculiar. In mammals, as a rule, the nerve is a smooth 
cylinder, proceeding straight to the sclerotic, penetrating the coats of the eye-ball directly, near 
the middle point behind, and then spreading out on the inside of the ball as a large circular 
concave mirror. This thin, saucer-like expansion of nerve -tissue is the retina. In birds the 
optic nerve is a fluted column, which approaches the eye-ball quite obliquely, strikes it at a 
point eccentric from the axis of the eye, and does not at once pierce the sclerotic. Tapering to 
a fine point, and running still obliquely, downward and forward, in a deep groove in the 
sclerotic that would be a tube were it not split, and through a similar slit in the choroid, a 
fluting of the nerve rises to attain the cavity of the eye, and the retina spreads out from the 
sides and end of this fold. But the prime peculiarity of a bird's eye is the purse " or comb," 
marsupium, pecten, f; a very vascular structure, like the choroid, and likewise painted black ; 
apparently erectile," that is, capable of increasing and diminishing in size by influx and efflux 
of blood. It is attached behind to the nervous structure ; is suspended in the vitreous humor, 
and runs forward obliquely a part or the whole of the way to the lens, to the envelope of which 
it may be attached in some cases. Its office is not fully determined. Its great resemblance 
the choroid proper suggests a similar function in the absorption of light. If it be turgid and 
flaccid by turns it must occupy a variable space in the vitreous humor, and in the former state 
press the waters upon the most yielding part of their walls, — that where the lens is situated, 
even to the extent of altering the position of the latter ; and if so, of changing the focus of the 
eye. It is difficult to account for the bird's eyes' powers of accommodation by the action of 
the ciliary muscle in only changing the shape of the lens, thus throwing out of account as 
impossible any change in the position of that refracting medium, or of the density of the 
refracting humors, or of the convexity of the cornea. The peculiar course of the optic nerve 
may be simply an anatomical convenience, or may have something to do with a bird's ability to 
see straight ahead though its eyes be laterally positioned. (See Am. Nat., ii, 1868, p. 578 ; Fi\ 
Bost. Soc. Nat. Hist., xii, Apr. 21, 1869.) 
Sense of Hearing : Audition. — This is enjoyed to a high degree by the musical class " 
of the Vertehrata, — birds being the only animals besides man whose emotions are habitually 
aroused, stimulated, and to some extent controlled by the appreciation of harmonic vibrations of 
the atmosphere. Most birds express their sexual passions in song, sometimes of the most 
ravishing quality to our ears, as that of the nightingale or the bluebird, and it cannot be sup- 
posed that they themselves do not experience the eff'ect of music in an eminent degree of 
pleasurable perturbations. Otherwise, they would cease to sing. The capability of musical 
expression resides chiefly in the more spiritualized male sex ; the receptive capacity of musical 
affections is better developed in the female, who chiefly furnishes the plastic material which is 
to be moulded into the physical manifestation of the male principle. Quickness of ear is 
extraordinary in such birds as those of the genus Mimus, which correctly render any notes they 
may chance to hear, with greater readiness and accuracy than is usually within human 
possibihty. It may be reasonably doubted that any others than some of the world's greatest 
musical composers have a higher experience in acoustic possibilities than many birds. Birds' 
ears have nevertheless a comparatively simple anatomical structure, on the whole much more 
like that of reptiles than of mammals. Such simplicity is seen in the ligulate or strap-shaped 
cochlea, the essential organ of hearing, figs. 84, 85, 86, 87, as compared with the helicoid curva- 
tion of the mammalian cochlea. The openness of the ear-parts which lie outside the tympanum 
is seen in fig. 62, at the place where the reference-lines ''ear-cells" reach the skuU ; and 
