NERVOUS SYSTEM. 23 



nivora, the Dolphin and most Apes. It is in this order of progres- 

 sion also that the number of lamellae and lobuli of the cerebellum 

 are developed, and that the ramifications of the arbor vitae and 

 the corpora rhomboidea within the olivary bodies, which form 

 indistinct external projections, become manifest. Above all we 

 meet with a pons Varolii, the size of which increases in the order 

 above given. One peculiar organ of the Mammalia, which does 

 not occur in Man, is the part called trapezium, a quadrangular 

 elevated layer of transverse medullary fibres, which lies close 

 behind the pons near to the pyramidal bodies, and abuts against the 

 origin of the auditory and facial nerves. The corpora quadrigemina 

 are usually very conspicuous, and very frequently provided with 

 an internal cavity ; they lie partly, as in the Marsupiata, the Eden- 

 tata, the Cheiroptera, and most Rodentia, perfectly free, and are 

 not reached by the hemispheres ; they are smallest in the Apes. 

 In the Carnivora, the posterior pair is as a rule the largest, in the 

 Ruminantia and Solipedia, the anterior. The thalami optici increase 

 in size inversely, in the ascending series of animals. The corpus 

 striatum is conspicuous, especially in the lower orders, and between 

 it and the optic thalamus there very generally occurs the broad and 

 often band-shaped stria cornea. 



In the cerebrum are found nearly all the parts of the human brain 

 repeated with certain modifications. The corpus callosum is very 

 rudimentary in the Marsupiata, and generally diminutive in the 

 Rodentia, Edentata, and Cheiroptera, where it extends but a very 

 little distance posteriorly. The corpora albicantia form as a rule 

 only a single mass, though sometimes, as in the higher Apes, they 

 are separated ; indications of their division are also found in the 

 Dog and other Carnivora. The pineal gland is always present, 

 varying in form and size, and, in those animals which have the cor- 

 pora quadrigemina uncovered, lies also for the most part freely ex- 

 posed, as, for example, in many of the Rodentia and Edentata. The 

 pituitary gland is of very considerable size, and situated upon a slen- 

 der infundibulum. The hemispheres exhibit the greatest number 

 of differences. The posterior lobes are either all but wanting, or so 

 very much shortened, that the cerebellum remains quite uncovered, 

 as in the Marsupiata, Rodentia, Edentata, and Cheiroptera. The 

 hemispheres are moreover quite flat, or have only a very few shal- 

 low depressions, as in Lepus and Cavia ; in the Insectivora also, as 

 Sorex, Talpa, Erinaceus, they are often altogether without sulci, and 

 in the Carnivora they frequently exhibit nothing more than a few 



