Comparative Anatomy of t/ie Brain. 583 



the ventral fissure. This fissure widens out in the sacrum, forming a 

 lozenge-shaped gash, or ventricle, which is denominated the sinus rhom- 

 boidalis. In the medulla oblongata there is another widening of the 

 fissure forming the fourth ventricle, as in the mammals, including man. 

 The enlargements of the spinal cord, opposite the fore and hind limbs, 

 are in proportion to the activity and power of the limbs, as we should 

 naturally expect. Thus, in flying-birds the enlargement at the part re- 

 lating to the wings is the larger one, while in the Runners, cursores, 

 Scratchers, rasores, and others which make much use of their legs, the 

 posterior enlargement is the greatest. 



The optic nerves of birds are usually very large. They originate 

 from the whole outer surface of the optic lobes, and from the optic 

 thalami as well ; the fibres from these two origins immediately joining 

 to form the nerve. A part of the fibres in each nerve remain on the 

 side on which they originate, and connect with the eye on that side, 

 while the rest " decussate," crossing over to the opposite side. After 

 the decussation the nerve is seen to be composed of longitudinal plates 

 piled one upon another ; doubtless they are plaits, or folds, of a flat 

 nerve, as in the fishes. ( Owen. ) 



It is worthy of observation that although birds are now toothless, 

 there are some, as the anatidce (ducks), and other water fowl, whose 

 beaks are notched or indented on the edges, and that to each of these 

 dentations several nerve filaments are distributed. These are from the 



FIG. 299. Embryo Chick 8 days. 



FIG. 300. " " 16 " 



FIG. 301. " " 20 " 



Letters same in all. 



O.-Olfactory lobes. 



A. Hemispheres of Brain. 



B Optic thalamus (fig. 299. 

 It is covered by the hemi- 

 spheres in figs. 300 and 301.) 



C. Optic lobes Corpora Bi- 



D . Cerebel lum . [ yemina. 



j. Fourth ventricle. 



^.Spinal cord. 



(John Anderson.) 



superior and inferior max- 

 illary branches of the fifth 

 pair. There is no gusta- 



FlG. 299. 



FIG. 300. 



FIG. 301. 

 tory branch to the tongue, that organ being one chiefly of prehension 



and not of taste, in birds. ( Owen. ) The sympathetic nerve s} T stem in 

 birds, partakes, in some particulars, of the character of that of rep- 

 tiles ; in others, of mammals. 



The brain of the embryo chick at eight days, shows the divergence of 

 the posterior p} T ramids and the formation of the fourth ventricle, the 

 rudimentary cerebellum at its upper end, and the corpora bigemina, or 

 optic lobes, and the hemispheres well started, and between them the 

 optic thalami (see fig. 299) lodged upon the crura cerebri. The optic 



