NERVOUS SYSTEM IN THE VERTEBRATA. 51 



in the larva state of those which undergo metamorphosis, as are 

 observed in the osseous fishes. The lobed form of the medulla 

 oblongata, the small cerebellum, the optic thalami, with the ventri- 

 cles, and the diminutive extent of the hemispheres, all evince a 

 degree of perfection not much above that noticed in fishes. The 

 spinal chord is prolonged, small and tapering into numerous coc- 

 cygeal vertebrae, and without sensible enlargements where the 

 nerves are to come off to supply the future members. The meta- 

 morphosis of the caducibranchiate species, from the pisciform to 

 the reptile state, develope an interesting series of phenomena. The 

 hemispheres become enlarged, the cerebellum, which was scarcely 

 visible, increases in size across the median line ; as the limbs begin 

 to appear, the spinal marow exhibits corresponding developments, 

 but dwindles posteriorly as the coccygeal vertebrae disappear. So 

 rapid are these changes in the nervous system of the frog, that we 

 can appreciate them from day to day. The sympathetic system is 

 more distinct in this class than in that of fishes. 



REPTILIA. 



In this class the cerebral hemispheres exceed in size the optic 

 lobes, and contain a distinct ventricle. The cerebellum is remark- 

 ably small ; and, in the sauria and chelonia, the spinal marrow 

 presents an obvious enlargement, opposite the attachment of each 

 nerve. The medulla oblongata is broad ; the nerves are large, 

 compared with the cerebral centres, but present no peculiarity of 

 distribution. The plexuses of the sympathetic are here more 

 closely connected with the arterial trunks, than in the preceding 

 classes. 



AVES. 



The brain and spinal chord are in this class developed with more 

 uniformity and perfection than in the cold-blooded reptilia ; and 

 bear a remarkable correspondence with the perfection of muscular 

 energy which they possess. In a pigeon weighing 3360 grains, 

 without the feathers, the brain weighed 37, and the spinal chord 11 

 grains=48. 



The spinal chord extends from the foramen magnum to the 

 coccygeal vertebrae, where it is greatly reduced in size, and expends 

 itself in distributing a few nerves through the lateral foramina. The 

 length of the chord is considerable, compared with the size of the 

 brain ; its shape is cylindrical ; its anterior and posterior grooves 

 are very distinct, as well as a minute canal extending through its 

 whole length, arising from the union of the two halves of the 

 chord: the dilatation of this canal in the pelvic region, is called 

 "rhomboidal sinus." The spinal chord is chiefly composed of 

 white matter, but contains a small quantity of gray, internally. 

 Two enlargements occur on the chord, bearing a relative size to 

 the development and powers of the extremities: in general the 

 posterior enlargement is the greater, especially when the business 



