THE ORGANS OF THE SENSES— RESPIRATION AND DIGESTION. 



presents both hemispheres of the larger brain, but not the convolu- 

 tions so characteristic of the brains of mammals. Birds, therefore, 

 in point of sagacity, are inferior to mammals, but superior to rep- 

 tiles, fishes, and insects. The bulk of the brain in proportion to 

 the size of the body varies in the different kinds of birds. In the 

 Eagle, it is i-26oth of the size of the body ; in the Sparrow, 

 i-25th ; in Jhe Goldfinch, i-27th ; in the Robin, i-6oth ; in the 

 Blackbird, i-68th ; in the Canary, i-i4th ; in the Duck, i-256th ; 

 in the Goose, i-36oth. 



The spinal nerve in the vertebras of the neck is round, and of 

 a uniform thickness. In the vertebrae of the back, it is broader 

 and thicker, but thinner in those of the pelvis. Otherwise the 

 nerves of birds, in their construction and distribution, greatly re- 

 semble those of mammals. 



The Organs of the Senses. 



All the organs of the senses in birds are well developed ; single 

 organs are simplified, but never entirely separated. 



The Eye is remarkable for its large size as well as for its curi- 

 ous construction. Its form and size vary greatly. All far-sighted 

 and nocturnal birds have very large eyes. Peculiar to a bird's eye 

 is the bony ring formed of twelve to sixteen four-sided scales, which 

 shove over each other like the shingles on a roof. They differ in 

 size, form, and strength. There is also a fan or comb, a closely 

 folded, dark colored skin, lying at the base of the vitreous body, 

 at the entrance of the nerve of vision, and very often extending 

 upward to the crystalline lens. The ring and fan enable the bird, 

 perhaps at will, to be far or near-sighted, and certainly they deter- 

 mine the unusual mobility of the eye. Besides the two eyelids, 

 birds possess a third, the nictitating or winking membrane. In a 

 bird that closes its eyes, the lower eyelid is drawn up to the upper, 

 differing in that respect from the human eye, for when the latter is 

 closed, the upper eyelid is drawn down to the lower. The nicti- 

 tating membrane is a curious appendage. When at rest, it lies in 

 the inner corner of the eye, toward the bill ; but, by the combined 

 action of two small muscles which are attached to the back of the 

 white of the eye (sclerotic coat), it can be so drawn out as to cover 

 the whole front of the eyeball like a curtain. This apparatus is 

 doubtless a very useful contrivance when the eye is exposed to a 

 brilliant light. 



Rapacious birds have the greatest range of sight. Spreading 

 birds, as Swallows, which catch insects on the wing, have a very 

 rapid flight, and an almost inconceivable quickness of sight. 



The iris varies in its color according to the age, sex, and kind 

 of bird. Brown is the prevailing color; but from this, the iris 

 passes through all shades to red, light yellow, or silver gray ; and 

 from the last to light-gray, and even blue. Some birds have a 

 bright green ; others, a bluish black eye. 



The Ears — An external ear does not exist in birds. The great 

 openings of the ears are located sideways on the back part of the 

 head, and are, in most birds, surrounded by loose bunches of feath- 

 ers, allowing the sound-waves to pass through without hindrance. 

 Owls have a substitute for an external ear, consisting of a fold of 

 the skin which the bird can open or shut at pleasure. The drum 

 skin lies close to the entrance ; the hearing passage is short and 

 membraneous, and the drum barrel is very spacious. Instead of 

 the three little hearing bones of the mammals, birds have one pol- 

 ygonous (many angled) bone, which has some resemblance to the 

 hammer. 



The Organs of Smell in birds are decidedly inferior to those 

 in mammals; The external nose and wide nostrils are want- 

 ing. The nostrils in birds are usually located in the upper mandi- 

 ble, near the root of the bill, and look like little round holes or 



slits, either bare or covered with a skin or bristle-like feathers. 

 The inside of the nose is divided into two cavities. In each of 

 these are three membraneous, cartilaginous or bony muscles, cov- 

 ered with a mucous membrane, on which are spread the nerves of 

 smell. 



It has been asserted that the sense of smell was developed to 

 the highest degree in the Vulturidae (Buzzards) ; but experiments 

 have clearly shown that it is the great development of the sense of 

 sight in these birds, which draws them so quickly to the spot where 

 a carcass is lying. But if the carcass is covered with a cloth, no 

 vulture will come near it, be its putrid smell ever so strong. Re- 

 move the cloth from only a small portion of the putrefying mass, 

 and the birds will come from a long distance for a feast. 



Taste. — Few birds have the tongue so constructed as to serve 

 for an organ of taste. Only the Parrot, Parroquet, and Loris, 

 have the tongue so constituted as to justify the inference that they 

 have the sense of taste. Their tongues are soft, thick, and cov- 

 ered with papilla?. A few of the Natatores (Swimmers) have a 

 similar tongue. It is, however, far inferior to the tongue of Par- 

 rots, as these appear to select their food by the sense of taste. In 

 most birds, the tongue is more or less separated, and is either short- 

 ened or diminished, or covered with a horny integument. In a few 

 birds, it is long and fleshy. It is probable that the tongue is used 

 by birds more as a feeler than as a taster, and that it may also 

 serve to extract and take hold of food. 



The Sense of Feeling, considered as mere feeling or as per- 

 ception, seems to be highly developed in birds, for the outer skin is 

 thickly set with nerves ; the tongue is often endowed with feeling, 

 and the bill covered with a soft skin. 



Respiration and Digestion. 



Very perfect in birds is the apparatus for the circulation of the 

 blood, and also that for respiration. 



The Heart and Lungs. — The heart of a bird has two cham- 

 bers, and two ante-chambers, similar to the arrangement in the 

 heart of a mammal, with this difference, however, that the mus- 

 cles of the bird's heart are comparatively stronger than those of 

 mammals. On both sides of the heart lay the lungs, which are 

 rather large in proportion. Sideways of the point of the heart, are 

 the two lobes of the liver. The lungs often adhere to the ribs, and 

 reach further down than in mammals. There is no division be- 

 tween the chest and the cavity of the abdomen, as the diaphragm 

 is wantingo All the great bones of the limbs, and many of the 

 body, communicate with the lungs, and are hollowed reservoirs of 

 air. There are, also, sacs, or bladder-like receptacles, which can 

 be filled with air, distributed about the body ; some of them in the 

 internal portions ; some between the muscles and the skin, along 

 the throat, the chest, and the sinews of the shoulders. These sacs 

 or bladders communicate with each other, and with the lungs. 

 The lungs can be only slightly expanded or contracted ; but as a 

 compensation for this disadvantage, the branches of the windpipe 

 open into the lungs, and these in turn communicate with the mem- 

 braneous sacs or air cells distributed throughout the entire body, 

 so that when air is blown into the windpipe of a bird, its whole 

 body becomes distended like a blown-up bladder. 



The Windpipe consists of bony rings combined with a strong 

 membrane, and possesses an upper and lower larynx. The former 

 lying behind the tongue, is nearly triangular, and has no cover; 

 its opening is surrounded by little warts, and lined on both borders 

 by a soft, muscular skin, by which the larynx can be completely 

 closed. The lower larynx lies at the end of the windpipe, just be- 

 fore the separation of the branches, and is a mere enlargement of 



