AVES. 



303 



the thorax, where it sends a considerable branch 

 from the first thoracic ganglion to join the pul- 

 monary plexus formed by the par vagum. 

 This ganglion also distributes seven other fila- 

 ments, one of which goes to join the brachial 

 plexus; a second is lost in the cardiac plexus of 

 the par vagum ; three other filaments proceed 

 inwardly to the projection formed by the bodies 

 of the vertebrae to produce the commencement 

 of the splanchnic nerve ; lastly, the sixth and 

 seventh serve to unite the first ganglion with 

 the second, one passing above, the other below 

 the head of the rib, which they thus include in 

 a lozenge-shaped space. Each of the succeed- 

 ing ganglions forms, in like manner, a centre of 

 nervous radiations, which are five, six, or seven 

 in number, of which four, two anterior and two 

 posterior, serve to bring the contiguous ganglia 

 into communication with each other; one or 

 two contribute to the formation of the splanch- 

 nic nerve, and one joins the dorsal spinal nerve 

 situated immediately behind the ganglion. 



The splanchnic nerves, formed by all the in- 

 ternal thoracic branches of the great intercostal, 

 accompany on either side the trunk of the 

 aorta. When it has arrived at the cceliac axis, 

 they surround it and form one, two, or three 

 ganglions from which an immense number of 

 filaments are thrown off, which surround the 

 different arteries of the abdomen. These gang- 

 lions are evidently the analogues of the semi- 

 lunar ganglions of man, and the filaments pro- 

 ceeding from them correspond to the solar 

 plexus. The trunk of the sympathetic con- 

 tinues along the bodies of the vertebrae, but 

 the ganglions become less marked after the ribs 

 cease to be given off; two or three filaments 

 are given off from each of these small swell- 

 ings, which, by uniting with the filaments of 

 the opposite side, form a plexus around the 

 aorta. The termination of the sympathetic may 

 be readily traced along the coccyx, where four 

 pairs of ganglions are observable in the Swan, 

 the last of which join to form a ganglion impar. 



Fig. 137. 



6* 



4* 



Cerebral nerves, eyes, fyc. in situ of a Goose. 



Organs of Vision. The eye in Birds pre- 

 sents many peculiarities, which chiefly relate to 

 the extraordinary powers of locomotion in this 

 class, tending to accommodate vision to a rapid 

 change of distance in the objects viewed, and 

 to facilitate their distinct perception through a 

 rare medium. 



There is no species of bird in which the eyes 

 are wanting, or are rudimentary, as occurs in 

 the other vertebrate classes. 



The eyes of Birds are, in the first place, re- 

 markable for their great size, both as compared 

 with the brain and with the entire head, (fig. 

 137,) being analogous, in this respect, to the 

 eyes of some of the flying insects. Their form 

 is admirably adapted to promote the objects 

 above named. The anterior segment of the 

 eye is more prominent than in any other class 

 of animals, and is in many birds prolonged into 

 a tubular form, terminated by a very convex 

 cornea (e,fig. 137.) Dr. Macartney observes 

 that " the owl furnishes the most striking ex- 

 ample of the disproportion between the anterior 

 and posterior spheres of the eye, the axis of the 

 anterior portion being twice as great as that of 

 the other. The obvious consequence of this 

 figure of the globe of the eye is to allow room 

 for a greater proportion of aqueous fluid, and 

 for the removal of the chrystalline lens from the 

 seat of the sensation, and thus produce a greater 

 convergence of the rays of light, by which the 

 animal is enabled to discern the objects placed 

 near it, and to see with a weaker light ; and 

 hence owls, which require this sort of vision so 

 much, possess the structure fitted to effect it in 

 so remarkable a degree." 



The anterior division of the eye is least con- 

 vex in the swimming birds. The sclerotic 

 coat is divisible into three layers. It is thin, 

 flexible, and somewhat elastic posteriorly, where 

 it presents a bluish shining appearance, without 

 any distinct fibres,but anteriorly its form is main- 

 tained by a circle of osseous plates or scales (f, 

 fig>\ 37) interposed between the exterior and mid- 

 dle layers. These plates vary from thirteen to 

 twenty in number, and are situated immedi- 

 ately behind the cornea, with their edges over- 

 lapping each other. They are in general thin, 

 and of an oblong quadrate figure, becoming 

 elongated from before backwards in proportion 

 as the bird possesses the power of changing the 

 convexity of the cornea. In the nocturnal 

 Raptores the bony plates are strong and thick, 

 and extend from the cornea over the whole of 

 the anterior projecting division of the eye to the 

 posterior hemisphere, which they also contri- 

 bute to form. The figure of the eye is thus 

 maintained, notwithstanding its want of sphe- 

 ricity; and in other classes, as Reptiles and 

 Fishes, where the eye recedes from the spherical 

 figure from an opposite cause, viz. the extreme 

 flattening of the cornea, that form is also pre- 

 served by the introduction of an osseous struc- 

 ture in the sclerotic. 



The bony plates are capable of a degree of 

 motion upon each other, which is, however, 

 restrained within certain limits by the attach- 

 ments of their anterior and posterior edges to 

 the sclerotic coat ; and by their being bound 



