178 
GENERAL ORNITHOLOGY. 
cranial bones. An extensive and intricate series of communications is effected with the nerves 
of the cerebro-spinal system, excepting the special-sense nerves of smell, sight, and hearing. 
The points of communication form a chain of sympathetic ganglia ; from these knots, the most 
conspicuous features of the system, nervous chords pass to their distribution in the motory 
mechanism of the heart and blood-vessels and other viscera. The anterior sympathetic nerves 
are the iridian ; the ganglia are the spheno-imlatine or meckelian, intimately connected with 
cranial nerves. The system ends behind in the caudal region of the spine by a ganglion 
impar. 
Sense of Smell : Olfaction. — The sense of smell is effected by terminal branches of the 
olfactory (1st cranial) nerve, ramifying in the mucous (pituitary or schneiderian) membrane 
of the nasal cavities. Owing to the comparatively small size and little complexity of the fold- 
ings and pleatings of bone or cartilage in the nasal chambers, the sensory surface being cor- 
respondingly limited, it is not probable that birds possess this sense in a high degree. Besides 
the cartilaginous or osseous septum, generally more or less complete in birds, there are lateral 
scrolls and whorls of bone iu endless diversity in most birds, which may be ossified, or remain 
gristly. The general cavity is mostly bounded and enclosed by the bony beak ; floored by the 
anterior part of the hard palate ; defended on each side by the descending prong of the nasal 
bone ; in the dry skull, it either seems continuous with the great orbital cavity on each side 
behind, or is separated therefrom by lateral ethmoid (pre-frontal) or lacrymal ossifications, or 
both. Outwardly the nasal chambers open upon the beak by the external nostrils — orifices of 
great zoological diversity, as already indicated (p. 104), bounded by prongs of the premaxillary 
and nasal bones. These openings are minute or quite obliterated in some Steganopodes, as 
pelicans and cormorants. The nasal cavities always communicate with the back part of the 
mouth, or the posterior nares (Lat. naris, a nostril) ; generally paired, that is, with a partition 
between them, sometimes united in one median aperture. The olfactory nerve, which is rather 
a prolongation of the rhinencephalon itself than an ordinary nerve, escaping from the brain- 
box by a special foramen, traversing the upper part of the interorbital septum in a groove or 
canal, enters the nasal cavity by a single orifice (excepting Apteryx and Dinornis), instead 
of the numerous apertures in a cribriform plate by which its filaments reach their destination in 
mammals. The true sensitive membrane in w^hich the nervous filaments end is that investing 
ethmoidal (septal and turbinal), not maxillary parts. An associate structure of the olfactory 
organ is the nasal gland, sometimes called the superorhital gland, from its position in many 
birds. Thus it is of great size in a loon, and lodged in large deep crescentic depressions on 
top of the skull over the orbits (fig. 63, iv) ; these crescents nearly meeting each other in the 
middle line. In other birds it is smaller, and within the cavity of the orbit, but never in that 
of the nose itself, its secretion being poured into the nasal chamber by a special duct. 
Sense of Sight: Vision. — The eye is an exquisitely perfect optical instrument, like an 
automatic camera obscura w^hich adjusts its own focus, photographs a picture upon its sensi- 
tized retinal plate, and telegraphs the molecular movements of the nervous sheet to the optic 
'Hwins" of the brain, where the result is biogenized; " that is, translated from the physical 
terms of motion in matter to the mental terms of consciousness. But no part of the nervous 
tract, from the surface of the retina to the optic centre, sees or knows anything about it, being 
simply the apparatus through which the Bird looks, sees, and knows. In this class of Verte- 
brates, the optic organs, both cerebral and ocular, are of great size, power, and effect ; their 
vision far transcends that of man, unaided by artificial instruments, in scope and delicacy. The 
faculty of accommodation, that is, of adjusting the focus of vision, is developed to a marvellous 
degree ; rapid, almost instantaneous, changes of the visual angle being required for distinct per- 
ception of objects that must rush into the focal field with the velocity at least of the bird's flight. 
