MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 331 



velopment as previous writers have affirmed, but is formed upon a distinct and 

 definite plan, and the variations that are met with are but modifications of this 

 plan. The development of the occipital lobe in the human fcetus can be traced in 

 Plate XXXV, figs. 8-17, and Plate XXXVI, figs. 1-4, according to the morpho- 

 logical type advocated above, and it will be readily seen, that it is in perfect 

 harmony with the results obtained by a comparative study of different Primate 

 brains. 



There still remains for us to discuss, the evolution of the superior mesial 

 occipital surface and the consideration of the relations and connections of the plis 

 de passage of Gratiolet; and it will be found that having once determined the 

 structure of the occipital lobe the difficulties and obscurities that have always 

 attended the study of these portions of the cerebral surface have almost com- 

 pletely disappeared. 



SUPERIOR MESIAL OCCIPITAL SURFACE. 



We will first consider the arrangement and relations of the mesial surface of 

 the occipital lobe as it is found in Man, and afterward compare it with the condi- 

 tions as found in the Simiadse. The mesial surface of the hemisphere as found in 

 the brain of a Negro is represented on Plate XLII, fig. 2. The calcarine fissure, 

 c a, will be seen extending backward to the end of the hemisphere where it ex- 

 pands into a transverse calcarine portion or extremity. By means of this fissura 

 calcarina the mesial surface of the brain is divided into two portions : an upper, 

 which proceeds anteriorly to the end of the frontal region, and a lower, which pro- 

 ceeds to the anterior extremity of the temporal. The portion below the calcarine 

 fissure or mesial occipito-temporal surface is found in Man similar to the same 

 region in the monkeys, the type of which we have already described, and need 

 not, therefore, consider at present. 



Above the calcarine fissure, however, and back of the position of the mesial 

 portion of the fissura occipitalis superior, we find in Man and some of the more 

 highly developed Simians, an increased development of what exists in the majority 

 of the monkeys as a far less developed portion of the mesial cerebral surface. 

 This is the wedge-shaped piece, c n, Plate XLII, fig. 2, the so-called cuneus of 

 Huschke, Ecker, and others. It is the lobule occipitale of Gratiolet, the internal 

 occipital lobule of Huxley, gyrus occipitalis primus, erste obere Hinterhauptlappen- 

 windung of Wagner, and the oberer Zwischenscheitelbeinlappen of Huschke. It 

 represents the mesial portion of the gyrus occipitalis superior as defined by the 

 writer. It is limited in Man by the parieto-occipital or mesial portion of the 

 occipital fissure uniting at an acute angle with the fissura calcarina. Its exact 

 meaning may be understood by a comparison of this region in a series of monkey 

 brains and applying the conclusions thus reached to the condition as found in Man 

 and the higher Simiadse. In Chrysothrix saureus, Plate XXXVIII, fig. 8, a 

 definite cuneus cannot be said to exist unless we consider the scroll-like folding of 



