SEC. IV ANATOMY OF BIRDS 273 



some cases. Its office is not fully determined. Its great resem- 

 blance to 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 humour, 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 slw^e, 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 

 humours, 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; Pr. 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 Vertebrata, — ^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 supposed that they themselves do not experience the effect of 

 music in an eminent degree of pleasurable perturbations. Other- 

 wise they would cease to sing. The capability of musical expression 

 resides chiefly in the more spiritualised 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 possibility. 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 com- 

 paratively 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 curvation 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 especially in Fig. 71, 

 where the stapes, st, is seen lying in the ear-cavity, the tympanum 

 having been removed. 



