90 AVES. 



Communications may be traced between the cranial portion of the 

 Sympathetic system and most of the cerebral nerves. The superior 

 cervical ganglion is of considerable size: The remainder of the main 

 trunk of the sympathetic then enters the uppermost cervicle verte- 

 brae by the canal formed by their double transverse processes, 

 communicates with the corresponding spinal nerves in that situation, 

 and forms ganglia upon the vertebral vessels. It next issues from 

 this canal, and is continued over the brachiar^plexus, to which other 

 ganglia developed in its course are very firmly attached, and at 

 length enters the thoracic cavity, where it becomes of very large 

 size and forms a ganglion in front of the head of each rib ; two 

 nervous filaments from each ganglion, one passing above, the other 

 below the neck of the rib, serve to bring into communication contig- 

 uous ganglia. The branches from the thoracic ganglia give off the 

 splanchnic nerve, which forms an aganglionic caeliac plexus accom- 

 panying the gastric and intestinal arteries, but passing chiefly to the 

 muscular parietes of the gizzard and to the liver. The nerve is then 

 prolonged to the caudal vertebrae, where it terminates in a series of 

 small ganglia. 



ORGANS OF THE SENSES. 



Organs of Vision. 



ALL Birds, without exception, are provided with a perfect and well- 

 developed visual apparatus. The eyes are always of great size, 

 being largest and most prominent in the Owls, flattened and smallest 

 relatively to the size of the head in the Natatores. They are but 

 slightly moveable, and are situated in orbitar cavities circumscribed 

 superiorly and laterally by bone, which are separated in the 

 cranium from each other by a thin osseous plate, frequently perfo- 

 rated to such an extent as to leave only an intervening membranous 

 partition. The globe of the eye, the form and component parts of 

 which may be recognised upon a perpendicular section, is convex 

 posteriorly, more flattened in front, or, as in the Owls, elongated into 

 a round cylinder, and in such cases provided with a very prominent 

 cornea. The very dense and tough sclerotic coat is supported by a 

 series of bony plates interposed between its two fibrous layers, and 

 united so as to form a continuous circular chaplet embracing the 

 posterior part of the eyeball as far forward as the margin of the 

 cornea ; their number consists mostly of 12 15, and they partially 



