88 AVES. 



coalesced into one with the third ventricle, and are prolonged for- 

 ward into the olfactory ganglia or mammillary processes, as in most 

 of the Mammalia. The fourth ventricle communicates as a narrow 

 slit with the stem of the arbor vitse, and in the direction forward with 

 the aqueduct of Sylvius. 



The Spinal Cord is connected to the brain through the interven- 

 tion of a well-developed medulla oblongata ; it is cylindrical through- 

 out, has an anterior and posterior fissure,- and a narrow central 

 canal which extends throughout its entire length. There is a 

 swelling upon the anterior extremity of the cord corresponding with 

 the origin of the brachial plexus of nerves ; another of larger size 

 is situated within the lumbo-sacral bone, and accords with the origin 

 of the nerves to the posterior extremities, while at its commence- 

 ment the lateral columns of the spinal marrow diverge so much as 

 to form a kind of groove, termed from its figure the rhomboidal sinus, 

 and which, communicating with the median canal of the cord, is 

 covered by a very soft gelatinous transparent substance, consisting 

 of an assemblage of delicate white cells permeated by blood-vessels. 

 The spinal cord then becomes narrower and is prolonged into the 

 canals of the caudal vertebrae. 



The twelve cerebral pairs of nerves in the class of Birds can be 

 readily referred to their types in the human subject. The nerves 

 of smell take their origin from a special swelling or olfactory gang- 

 lion. The optic nerves are always of very large size and cylindri- 

 cal, and form a complete union or chiasma, which consists in gene- 

 ral of eight nervous fibrils, that decussate each other. The fifth pair 

 is very conspicuous, and the larger or sensitive root has a Gasserian 

 ganglion upon it. Its first or ophthalmic division combines with 

 a branch from the oculo-motor, or third pair of nerves, to form a 

 large ciliary ganglion, from which is given off directly the ciliary 

 nerve supplying minute filaments to the iris. No connexion of this 

 nerve with the sympathetic has been observed ; a fact which is the 

 more interesting, from the relation which it probably bears to the 

 voluntary power possessed by Birds over the movements of their 

 iris. From the ophthalmic division of the fifth a branch proceeds 

 also to the Harderian gland, another to the nasal organs, while a 

 third ramifies upon the beak, so that this nerve is more extensively 

 distributed than in the Mammalia. The second division or supe- 

 rior maxillary nerve is particularly large in the Ducks and Geese, 

 and ramifies in the upper mandible beneath its marginal lamella? 

 and the mucous lining of the palate ; branches also arise from it to 



