SPINAL MARROW. 149 



serrations which have been made on this subject, are too 

 imperfect to enable us to determine, with precision, the pe- 

 culiar functions of the different parts, or the effects which a 

 modification of the form or situation of these parts is calcu- 

 lated to produce. Before taking any farther notice of this 

 part of the subject, let us attend to the actual differences of 

 form and structure exhibited by the different classes of ani- 

 mals. 



II. Varieties of Structure in different Animals. 



From an examination of the brain of quadrupeds, birds, 

 reptiles, and fishes, M. CUVIER draws the following conclu- 

 sions, which mark the peculiar features of each of these 

 classes. 



" 1 . The character which distinguishes the brain of Mam- 

 malia from that of the other red-blooded animals, consists, 

 " a. In the existence of the corpus callosum, the fornix, 



the cornua ammonis, and the pons Varolii. 

 *' &, In the tubercula quadrigemina being placed upon 



the aquaeductus Sylvii. 



** c. In the absence of ventricles in the optic thalami, andin 

 the position of these thalami within the hemispheres. 

 46 d. In the alternate white and grey lines within the cor- 

 pora striata. 



" 2. The character peculiar to the brain of Birds, consists 

 " a. In the thin and radiated septum, which shuts each 



anterior ventricle on the internal side. 

 " 3. The character of the brain of Reptiles depends 

 " a. On the position of the thalami behind the hemi- 

 spheres. 



" 4. The character belonging to the brain of Fishes, con- 

 sists, 



" a. In the tubercles of the olfactory nerves, and the tu- 

 bercles situated behind the cerebellum. 



