184 GENEBAL ORNITHOLOGY. 



cranial bones. An extensive and intricate series of communications is effected with the nerves 

 of the cerebro- spinal system, excepting the special-sense nerv^es of smell, sight, and hearing. 

 The points of communication form a chain of sjTnpathetic 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-palatine or meclcelian, 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 schneideriau) 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, tliere are lateral 

 scrolls and whorls of bone in 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. 109), bounded by prongs of the premaxillary 

 and nasal bones. These openings are minute or quite obliterated in some Steganopodes, a& 

 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 cribrifonn plate by which its filaments reach tlieir destination in 

 mammals. The true sensitive membrane in which the nei-\'ous 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 superorbital 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 skuU over the orbits (fig. 63, w) ; 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 which 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 

 ''twins" of the brain, where the result is '' biogenized;" that is, translated from the physical 

 terms of motion in matter to the mental tenns 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. 



