52 



of progression devolves on the posterior members, as in strnthious 

 birds. The form of the chord is not altered in the alar enlarge- 

 ment, but simply increased by an accession of gray and white 

 substances; — the lower one, on the contrary, not only receives 

 additional matter, but the pillars separate, so that the fluid in the 

 sinus is merely covered by pia mater. 



The brain of the bird differs from that of the reptile in the 

 greater size of the cerebrum, and the more complex structure of 

 the cerebellum; it differs from the brain of a mammal in the 

 smaller size of the cerebellum, and the rudimentary state of the 

 fornix; and it differs from the brain of every other vertebrate class 

 in the inferior position of the optic lobes. The cerebral hemispheres 

 are generally of a convex condiform shape, with the apex directed 

 forwards; they are disunited through their whole extent, being 

 only joined by a round anterior commissure; they are destitute of 

 convolutions, and have a small ventricle in their interior. The 

 olfactory tubercles are greatly reduced in size, and retain their 

 tubular communication with the cerebral ventricles. The optic 

 lobes are small ; gray on the surface, white internally, and contain 

 each a small ventricle ; they are connected by transverse medul- 

 lary bands on which the pineal gland rests, with its peduncles 

 directed forwards over the optic thalami. These last bodies are 

 covered by the cerebral hemispheres, united by a commissura 

 mollis, and destitute of transverse sulci. The medulla oblongata 

 is large and wide; its components are marked on the surface, but 

 it is without tuber annulare. The cerebellum presents a median 

 vermiform lobe, and rudimentary hemispheres, sulcated trans- 

 versely, with a faintly-marked arbor vitae in the interior. The 

 membranes investing the brain differ but little from those of the 

 mammalia. 



Nerves. — The nerves in this class present but few striking pecu- 

 liarities, being distributed nearly as they are in man. The olfactory 

 arise from the front of the hemispheres, pass forwards through 

 distinct osseous canals in the cribriform plate, and are distributed 

 in a radiated manner on the superior spongy bone. The large 

 optic nerves arise from the optic tubercles, and form a perfect 

 decussation in front of the infundibulum ; where, as in the iguana, 

 an incision displays a mutual intermixture of the fibres. The 

 remaining cerebral nerves are distributed pretty nearly as in mam- 

 malia ; the portio dura is, however, small in accordance with the 

 insensibility of the superficial parts of the face. The spinal nerves 

 correspond in number to the vertebrae ; they arise by two roots, the 

 posterior having a large ganglion on it. The sympathetic, which 

 is well developed, communicates through the anterior lacerated 

 opening with the fifth and sixth nerves. From the third cervical 

 vertebra to the thorax, it is contained in the canal in the transverse 

 processes, in company with the vertebral artery. It form a series 

 of ganglions, from the base of the skull to the end of the coccyx ; 

 and communicates, in its course, with every neighbouring nerve. 



