This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2075). The sense of touch must obviously be extremely imperfect in these animals: their body, enveloped in feathers, can be little sensible to impressions produced by the contact of external objects; and their limbs, covered as they are with plumes, or cased in horny scales, are but little adapted to exercise the sense in question. The beak alone offers itself as calculated to be a tactile instrument; but even this, enclosed as it is in the generality of birds by a dense corneous case, must be very inefficient in investigating the outward surfaces of substances; nevertheless in some tribes the beak is undoubtedly extremely sensitive, and is used to search for food in marshy soils, or to find it in the mud at the bottom of shallow waters. This is the case, for instance, in many of the long-billed Wading Birds, and also in the flat-billed aquatic families, such as the Goose and Swan; in these, in fact, the covering of the beak is comparatively soft, and the nerves that supply it, derived from the fifth pair, are of very considerable size.
(2076). As we advance from the lower to the more highly gifted races of the animal creation, taste is evidently one of the last indulgences granted; and even in Birds it is only necessary to inspect the structure of the tongue in order to be convinced that they can derive but small enjoyment from this source. The skin of the tongue in these creatures is totally devoid of gustatory papillae, and frequently, indeed, enveloped in a horny sheath; so that, if the sense of taste exists at all, it must be to the last degree limited and obtuse.
(2077). In return, however, for the imperfection of the above senses, the olfactory apparatus in this class of animals begins to assume far greater importance than in the cold-blooded Vertebrata; and the nasal cavity indicates, by its extent, that it is now well adapted to investigate the odorous properties of the air taken in for respiration. The septum narium completely divides the nose into two lateral chambers of considerable extent, which individually communicate with the pharynx (fig. 363, c); and, upon the outer wall of each compartment, three convoluted lamina3, covered with a most delicate Schneiderian membrane, represent the turbinated bones of Mammalia, and increase the olfactory surface. Of these, the middle turbinated bone (fig. 363, a) is the largest; but the superior appears to be the most important, as it is upon this that the olfactory nerve is principally distributed, insomuch that Scarpa considered that the comparative powers of smell possessed by different birds might be estimated by the development of this portion of the olfactory organ. The olfactory nerves (fig. 363, b), as in Reptiles, still enter the nose without dividing, so that there is no cribriform plate to the ethmoid bone. The nostrils are simple apertures, perforating some part of the horny beak covering the upper mandible, and are never provided with moveable cartilages or muscles, as those of Mammalia will be found to be.
Fig. 363. Olfactory apparatus in a Goose.
(2078). The eye of a Bird is an optical instrument of such admirable construction, that, did not the nature of this work compel us to adopt the strictest brevity in our descriptions, it might well tempt us to indulge in lengthened details relative to the adaptation and uses of its various parts. If we contrast the Bird with the Reptile, or more especially with the Fish, and consider the totally different circumstances under which these animals exercise the sense of vision, we may well expect extraordinary modifications in the structure of their organs of sight. The Fish, immersed in a dense medium, can see but to a very limited distance around it; and the sphericity of the crystalline lens, with the consequent contracted antero-posterior diameter of the eyeball, at once testifies how small is the sphere of vision commanded by the finny tribes. The Bird, on the contrary, dwelling in the thin air, and not unfrequently soaring into regions where that air is still further rarefied, must survey a horizon even more extensive than that enjoyed by the terrestrial Mammal; while, from the rapid movements of the feathered races, it becomes absolutely requisite that the focus of the eye shall continually vary between the extremes of long- and shortsighted vision.
The birds of prey, as they fan the air at an altitude which places them almost beyond the reach of human sight, or sail in broad gyrations through the sky, are scanning from that height the surface of the ground, and looking out for mice or other little animals on which to feed: but when the prey is seen, and the bird, shooting down with the rapidity of a thunderbolt, stoops upon the quarry, it must obviously be indispensable that it should see with as much clearness and distinctness when close to its victim, as it did when far remote; and to enable it to do this, special provisions have been made in the structure of the eyeball.
(2079). A glance at fig. 365, exhibiting a section of the eye of an Owl, will show the anatomist that, in its general composition, the organ is similar to that of Man. The sclerotic and the choroid tunics present the same arrangement, the transparent humours of the eye occupy the same relative positions, and the iris and ciliary folds exist, as in the human subject. Descending from generalities, however, he will find many points in the organization of a bird's eye eminently deserving separate examination; and it is to these we would specially invite his notice. First, the shape of the eyeball is peculiar: it is not spherical, as in Man, nor flattened anteriorly, as in Fishes and aquatic Reptiles; but, on the contrary, the cornea is rendered extremely prominent, and the anteroposterior axis of the eye considerably lengthened. This is remarkably exemplified in the Owl, in which bird, as Dr. Macartney * pointed out, such is the disproportion between the anterior and posterior spheres of the eye, that the axis of the anterior portion is twice as great as that of the other.