204 



THE POPULAR EDUOATOE. 



ANIMAL PHYSIOLOGY. VI. 



THE EAE (concluded). 



WE have to search for the orifice of the ear of birds beneath 

 the feathers. In a few cases, as in the owl and wild turkey, a 

 circle of feathers surrounds the ear-hole ; but generally there is 

 no external indication of an ear. On examination, however, a 

 zone of fine feathers with peculiarly fine barbs, through which 

 the air passes readily, is found round the ear. Internally, the ear 

 is not unlike that of brutes, except in the following particulars. 

 The cochlea is rudimentary, that is, it is not developed into a 

 coiled double canal, but is only a slight process from the vesti- 

 bule, occupied by two cylinders of fine cartilage, representing 

 the two staircases, but of very simple form. The semi-circular 

 canals are similar and similarly disposed, except that two of 

 / them, the horizontal and one of the vertical ones, communicate 

 where they cross one another. The most marked difference is 

 that the chain of ossicles is reduced to one long one, forked at 

 its tympanic end, and stretching right from the membrane of 

 the oval hole to the cartilage of the drum-membrane. The 

 whole organ is very compact and embedded in bone, and even 

 the canal whieh runs from the tympanum to open at the top of 

 the throat is of bone. 



No one can doubt that the sense of hearing in birds is keen 

 and appreciative. Indeed, if the correlation between the capa- 

 bility of producing a variety of sounds and the appreciation of 

 the same be as close as we should naturally suppose it to be, 

 the sense of hearing in our song birds is most exquisite. If the 

 hen nightingale experiences a corresponding happiness in listen- 

 ing to the song of her mate to that wliich he evidently feels 

 while his little throat is pouring forth its changeful notes ; or if 

 either of them can appreciate the impressions produced by the 

 varied music, ranging as they do from a sweet melancholy to a 

 thrilling joy, then these little summer visitants have an avenue 

 to a constant pleasure, and by the possession of this they make 

 a nearer approach to us than we have been disposed to admit as 

 possible to any of the lower animals. That such should be the 

 case may seem in the highest degree improbable to some minds ; 

 yet, before it is dismissed as a sentimental fancy, it should be 

 remembered that our greatest naturalists hold it as a principle 

 that a species is endowed with no habit or instinct, no product 

 or power, which is solely for the benefit of other species in 

 fact, that the primary use of every such endowment is for the 

 advantage of the species which possesses it ; and if in the great 

 harmony of Nature other species benefit from it, this is inci- 

 dental, though not accidental. The bee makes honey for its 

 own community, though man and the brown bear despoil its 

 comb. Though leather and fur are so useful and almost indis- 

 pensable to us, they were more useful and wholly indispensable 

 to the beast that they once clothed. By analogy, therefore (to 

 which there is no counter analogy), when we listen delighted to 

 the strains of the nightingale in May, we may infer that the 

 brooding bird experiences a yet more exquisite delight. So 

 general is this principle, that it is considered certain that every 

 species which produces sounds for its own sake, and dis- 

 connected with other necessary movements of the body, also 

 possesses an organ of hearing. 



The class of cold-blooded animals called reptiles, and which is 

 ill represented except in the tropics, contains creatures of very 

 different structure. The higher of these animals are more like 

 to birds than to the lower members of their own class, and these 

 again have a close resemblance in some respects to fish. Hence, 

 as in the case of the eye, the ear of a typical reptile cannot be 

 described as the ear of the class, because there are such great 

 differences in this organ. Thus, the ear of the crocodile is 

 almost precisely like that of a bird, and it is only in the means 

 of letting the air into the -tympanic cavity that there is much 

 difference. The crocodile while it lives in the water breathes 

 air, and it is provided with a means of drowning its prey under 

 water while it is itself inhaling the air. This is effected by the 

 channels of the nostrils being carried far back before they com- 

 municate with the throat, while a double valve in front of the 

 communication closes and cuts off the throat from the mouth. 

 By holding the prey crossways, and far back against the corners 

 of its widely-gaping jaws, it keeps it under water while its own 

 long snout and nostrils are thrust above the surface. Now we 

 have seen that the tympanic cavity must be supplied with air, and 

 water moist be excluded from it ; hence the Eustachi&n tube, or 



rather complicated system of tubes, is carried backward instead 

 of forward, and opened by a single orifice, behind the hind opening 

 of the nostrils, into the throat, and therefore behind the valve, 

 and the opening is on a projection and closed by a half -moon- 

 shaped valve. Every precaution is thus supplied to exclude the 

 water from, and include the air in, the tympanic cavity. Lizards, 

 turtles, and also frogs, have a drum and drum-membrane ; but 

 this is on a level with the rest of tho skin, so that there is no 

 ear-hole, and in the case of the turtle the drum-membrane is 

 covered by that hard scale which is next but one above the 

 corner of the mouth. In the serpent there is no drum-membrane 

 or air cavity ; the long bone running from the oval hole through 

 a cellular substance, and fastened by cartilage to the sealy 

 skin ; while in some of the lower fish-shaped reptiles the oval 

 hole is brought right up to the surface, to the exclusion of drum, 

 drum-membrane, and ossicles. 



In tracing the organ, then, throughout this class, we have 

 gradually lost all the outer courts of the ear, and also what 

 remnant of a cochlea was left. 



In the bony fishes all these parts are wanting, as might be 

 supposed ; but the ear, instead of being brought to the surface, 

 is walled up by the bones of the large skull. If the roof of the 

 skull of a fish be removed, a central compartment will be seen, 

 much too large for the small brain, and on either side, at the back 

 part, a large chamber, which communicates with the central one, 

 and in which the large main portion of tho ear is lodged ; while 

 the three semi-circuiar canals springing from this part by dilated 

 bags run, two of them upward into tubular hollows ef the skull- 

 bones, and then unite to run into the same vestibular sack 

 by a more central communication, while the third is horizontal 

 and runs outward. Tho main vestibular sack has itself several 

 compartments which sometimes communicate with it only by 

 narrow constricted necks, and in these are found the otolites, or 

 ear-stones, which are suspended over the parts to which the 

 strands of the ear-nerve are most largely distributed. These 

 ear-stones are no longer fragmentary particles, as in tho case of 

 mammals, or soft chalk, as in the internal ears of frogs, but 

 dense, hard, pearly bodies, one of which is of large size, and is 

 represented in the engraving with its concave, streaked side 

 towards the observer, this side being upward when in its natural 

 position. 



In illustration of what has been said concerning the advantage 

 of causing the sound waves to be reverberated in air, a peculiar 

 connection between the labyrinth and the internally situated 

 air-bladder of some fish ought to be mentioned. In the carp, 

 each ear-sack sends a passage to a central cavity in the base of 

 the skull, and this has two bags at its hind end, all filled with 

 fluid, as the cavity of the ear is, and from these a chain of three 

 bones runs to the bladder. In the little fish called the loach, 

 which is one of the first captives obtained by the searcher of 

 the little pools left by the retreating tide, the air-bladder seems 

 to be retained solely to minister to the ear ; and in the herring 

 the bladder itself sends processes to be applied to other pro- 

 cesses sent to meet them from the vestibule. 



In the other great order of fish distinguished from the fore- 

 going ones by the general character of the skeleton, this being 

 not bony, but grisly from the fact that elastic cartilage is not 

 so resonant a body, and not so good a conductor of sound, as 

 bone, other appliances are given to bring the ear in closer 

 relation to the external water, whence the sounds come. Tho 

 whole labyrinth is closely surrounded by gristle, and in sharks 

 from the gristle cavity a canal runs to the top of the head, and 

 is there closed by the skin. In the ray, a canal runs from the 

 union of the two semi-circular canals to a similar orifice. Both 

 of these canals are of course filled, not with air, but with fluid, 

 that of the shark being filled with what is called perilymph, or 

 external fluid, and that of the ray with endolymph, or internal 

 fluid. 



So much has been conjectured, and so little is really known, 

 about the organ of hearing in the invertebrate classes, that it 

 is scarcely advisable to enter upon the subject in a popular 

 publication. The great diversity of sounds produced by insects, 

 some of which, like the cicada (which makes the Italian coppices 

 ring perpetually with its loud, grating cry), have very elaborate 

 contrivances for the production of noises, makes it almost certain 

 that this large order of the jointed animals have the sense of 

 hearing. On the other hanov the almost universal muteness of 

 the mollusca might have led ua to suppose that the organ of 



