| Fig. 16.—Auditory “chain” of Chicken. Lateral and basal views. (After W.K. Parker). |
The middle ear communicates with the mouth by the Eustachian tubes, which pass between the basisphenoid and basioccipital bones, and unite upon the ventral side of the sphenoid, a little behind its articulation with the pterygoids, where they open into the mouth cavity by a short membranous duct. The columellar apparatus, or auditory chain of ossicles (fig. 16), extending between the fenestra ovalis and the tympanic membrane or drum, consists of (1) the long and slender columella, a straight, ossified rod which fits with a disk into the fenestra ovalis; it is homologous with the stapes (m.st.), although not stirrup-shaped; (2) the extracolumellar mass. This is chiefly cartilaginous and sends out three processes: the dorsal (s.st.) is attached to the upper wall of the drum cavity; the outermost (e.st.) is fastened on to the middle of the drum membrane; the third, ventral or infracolumellar process (i.st.) is directed downwards and tapers out into a thin, partly cartilaginous, strand, which originally extended to the inner corner of the articular portion of the mandible, but on its long way comes to grief, being squeezed in between the pterygoid and quadrate. This long downward process being homologous with an almost exactly identical arrangement in the crocodile, and with the processus folii of the mammalian malleus, it follows that the whole extracolumellar mass, that between stapes and drum, is equivalent to incus and malleus of the mammalia. There is, in birds, no annulus tympanicus. Birds possess an ear-muscle which at least acts as a tensor tympani; it arises near the occipital condyle, passes through a hole into the tympanic cavity, and its tendon is, in various ways, attached to the inside of the membrane and the neighbouring extracolumellar processes.
As regards the inner ear, the endolymphatic duct ends in a closed saccus, imbedded in the dura mater of the cranial cavity. The apex of the cochlea is turned towards, and almost reaches the anterior wall of the occipital condyle; at most it makes but half a twist or turn; it possesses both Reissner’s membrane and the organ of Corti. Although the scala tympani is so rudimentary, not reaching a higher level than in most of the reptiles, and remaining far below the mammalia, birds do not only hear extremely well, but they distinguish between and “understand” pitch, notes and melodies.
See G. Breschet, Recherches anatomiques et physiologiques sur l’organe de l’audition chez les oiseaux (Paris, 1836), with Atlas; C. Hasse, various papers in Zeitschr. f. wiss. Zool. vol. xvii, and in Anatomische Studien, pts. ii. and iv. (Bresku, 1871); I. Ibsen, Atlas anatomicus auris internae (Copenhagen, 1846); G. Retzius, Das Gehororgan der Wirbelthiere (Stockholm, 1884), ii. pp. 139-198, pls. 15-20.
Nose.—The olfactory organ is poorly developed, and it is still a question whether birds possess much power of smell; many are certainly devoid of it.
The olfactory perceptive membrane is restricted to the posterior innermost region of the nasal chamber, where it covers a slight bulging-out prominence on the nasal wall. This so-called third, upper or posterior conch is not a true conch, nor is that of the vestibulum; only the middle one forms a scroll, and this corresponds to the only one of reptiles and the lower of the mammals. The nasal cavity communicates with the mouth by the choanae or posterior nares, situated between the palatine process of the maxillary, the palatine and the vomer. The outer nares or nostrils are most variable in size and shape. In the Steganopodes they tend to become much reduced, e.g. in cormorants (Phalacrocoracidae), and especially in Sula, where the nasal slits become completely closed up, and the greater portion of the nasal cavity is also abolished, being restricted to the olfactory region with its unusually wide choanae. The nasal septum is often more or less incomplete, producing nares peniae, e.g. in the Cathartae, in the Anseres, gulls, rails and various other aquatic birds. The secretions of the mucous membrane of the nasal cavity, and a pair of naso-lacrymal glands (not to be confounded with the Harderian and the lacrymal glands), moisten and clean the chamber. The glands are variable in size and position; when very large, e.g. in plovers, they extend upon the forehead, causing deep impressions on the bones of the skull. Jacobson’s organ has been lost by the birds, apparently without a trace in the embryonic fowl, but T.J. Parker has described vestiges of the corresponding cartilages in the Apteryx (Phil. Trans., 1890).
See C. Gegenbaur, “Über die Nasenmuscheln der Vögel,” Jena Zeitschr. vii., 1873, pp. 1-21.
5. Vascular System.
The heart lies in the middle line of the body, its long axis being parallel with that of the trunk. The whole ventral surface of the pericardium is exposed when the sternum is removed. The right and left halves are completely divided by septa, no mixture of the venous and arterial blood being possible, an advance upon reptilian conditions, even the highest.