184 GENERAL ORNITHOLOGY. 



owing to the relatively greater size and convexity of tlie cdrnea. Tlie waters are eneloscd in 

 exceedingly delicate; membranes ; the vitreous in the hyaloid weinbrane, e, which, besides 

 lining the posterior chamber and enclosing the lens as already said, sends thin partitions all 

 through the vitreous Immor to steady these glassy waters. 



The optic nerve, a, of birds is peculiar. In mamnuils, as a. rule, the nerve is a smooth 

 cylinder, proceeding straight to the sch>r(jfic, penetrating tlu' coats of the eye-ball directly, near 

 the middle point bc^hind, and then sjin-ading t)nt on the inside of tlie ball as a large circular 

 concave uiirror. This thin, saucer-like expansion of nerve-tissue is the retina. In birds the 

 optic nerve is a tinted column, which apjiroaches tlu' eye-ball ((uite obli(|nely, strikes it at a 

 ])oint eccentric from the axis of the eye, and does not at tmce pierce the sclerotic. Tapering to 

 a hue jHiiiit, and ruuinng stiU obliquely, downward and forward, in a deep groove in tlic 

 sclerotic that would be a tube M'ere 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 ]iriuu- peculiarity of a bird's eye is tlm " purse " or " comb," 

 marsupi'wm, pccteit, f: a very vascular structure, like tlie choroid, and likewise painted black; 

 .•ipparently " erectile," that is, capalih' of incieasing and (hiiiinisliiui; in size by influx and efflux 

 of blood. It is attached behind to the nervous structure; is suspended in tlie vitreous humor, 

 and runs forward obliquely a part or the whole of llie way to the lens, to the env(dope of which 

 it nuiy be attached in souui ca.ses. Its office is not tally deti'rmined. Its great resemblance to 

 the choroid proper suggests a similar function in the absorption of liglit. If it be turgid and 

 flaccid by turns it nurst occupy a variable space in tlie vitnaius 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 thi' hitter; and if so, of elianghig the focus of the 

 eye. It is difficult to account for the bird's eyes' jiowers of accommodation Viy the action of 

 the ciliary muscle in only changing the sliitpc of the lens, thus throwing out of account as 

 impossible any change in the position of that refracting mi^dium, or of the density of the 

 ri'frac.tint; liumors, 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 i^es be laterally positioned. (See Am. Nat., ii, 1868, p. 578 ; I'r. 

 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 a]qireciatioii of harmonic vibrations of 

 the atmosphere. Most birds express their .sexual passions in song, sometimes of the most 

 ravishing ((uality to our ears, as that of the nightingah' or the bluebird, and it I'aiinot be sup- 

 posed that they themselves do not experience the eft'ect of music in an eminent degree of 

 pleasurable jierturbations. Otherwise, they vv'ould cease to siiii;. Th(! cajiability of musical 

 ex|iressiou resides chiefly in the more spiritualized male sex ; the recejitive capacity of musical 

 affections is bettor developed in the female, wlio chiefly furnishes the jdastic material which is 

 to be moulded into the physical manifestation of the male piinci]de. Quickness of ear is 

 extraordinary in such birds as those of the genus Mimus, which correctly render any notes tliey 

 may chance to hear, with greater readiness and accuracy than is usually within human 

 possibihty. It maybe rea.sonabIy doubted that any others tlian some of the world's greatest 

 musical compos(!rs have a higher experience hi acoustic iiossibiiities than many birds. Birds' 

 ears have nevertludess 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 

 (Mjchlea, the essential organ of hearing, figs. 84, 8.5, 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 skull; and 



