322 SPECIAL PHYSIOLOGY. 



with a thin layer of cerebral substance ; in these lowest Vertebrata, the parts 

 of the encephalon are, so to speak, analyzed, being arranged in a series of three 

 pairs of ganglionic masses, placed in a double symmetrical row, one behind 



Fig. 64. 



Fig. 64. Views of the upper surface of the brains of a fish, amphibian, bird, and mammal, showing the 

 gradual increase in the size of the cerebral hemispheres, a, brain of the codfish, showing, from behind 

 forwards, part of the spinal cord, the back of the medulla oblongata, the median cerebellum, the two 

 large optic lobes, the small cerebral hemispheres, consisting chiefly of the corpora striata, the narrow ol- 

 factory lobes and olfactory nerves, and lastly, the decussating optic nerves, b, brain of the frog, in which 

 the cerebral hemispheres are the largest masses; the olfactory lobes are seen in front, and the optic lobes, 

 projecting laterally behind ; the cerebellum is a thin transverse lamina, c. brain of the pigeon; the largest 

 masses are the cerebral hemispheres. In front are seen the ends of the olfactory lobes; behind, the cere- 

 brum, and projecting at the sides, are the optic lobes and corpora quadrigemina; and, in the middle line, 

 the laminated central lobe of the cerebellum, with its small lateral appendages, rf, brain of the rabbit, 

 showing in front, the large olfactory lobes; next behind them, the cerebral hemispheres, pointed in front, 

 slightly sulc-ated, and now, by far the largest mass of the encephalon: behind them, the cerebellum and its 

 floccular appendages; and lastly, the back of the medulla oblongata. 



the other, and of a single median mass behind them. The anterior pair of 

 these ganglionic masses, form the olfactory lobes ; the second pair, named the 

 cerebral lobes, are composed of the corpora striata, covered with a thin layer of 

 cerebral substance, which forms the rudimentary cerebral hemispheres ; the 

 third pair, usually the largest, correspond with the optic thalami and corpora 

 quadrigemina, and, as they give origin to the optic nerves, they are called the 

 optic lobes. Behind them is the small median mass representing the cerebellum. 



In Fishes, therefore, the thin cerebral hemispheres must fulfil a very sub- 

 ordinate office in the nervous functions ; in Amphibia, Reptiles, and even in 

 Birds, they are still small, and their component gray matter is of but little 

 thickness ; but so largely developed as they are in the Mammalia generally, 

 especially in the highest forms, and, above all, in Man, they appear, as al- 

 ready mentioned, more like superadded parts, overlapping all the other en- 

 cephalic masses. 



The progressive complexity of surface, of the cerebral hemispheres, is indi- 

 cated by their smoothness in Fishes, Amphibia, and Reptiles ; and by their 

 faintly marked sulcation in certain birds. In the lowest Mammalia, and even 

 in the smallest and lowest genera of the highest order of Mammals, the hemi- 

 spheres are also smooth, or nearly so, as, for example, in the Monotremata 

 and Marsupialia, in the lowest Rodentia, and even in certain lemurs, the low- 

 est of the so-called Quadrumana. As we advance in the ascending series of 

 Mammalia, or in the ascending series of genera in certain Orders, the hemi- 

 spheres become more and more sulcated on the surface, and finally, are mod- 

 elled into the curved or tortuous ridges called convolutions, which, speaking 

 generally, become more and more numerous and complex, as we reach the 

 highest Mammalia, or the highest genera in the several Orders. 



These cerebral convolutions, which may be said to be peculiar to the brains of 

 Mammalia, may be considered first, in reference to their general plan in any 

 given group or groups ; and secondly, as regards their relative complexity 

 within that plan. In the Pachydermatous and Ruminant animals, for ex- 

 ample, the convolutions are chiefly arranged in the form of parallel folds, 

 extending from the front to the back of each hemisphere ; and, for the most 

 part, present a more or less flexuous outline. In the Carnivora, on the other 

 hand, the surface of the hemispheres, is divided into four principal antero- 

 posterior convolutions, which seem to bend, in simple curves, around the upper 



