MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 309 



without difficulty. Sometimes, however, they are obscured by crossing or bridging 

 convolutions. I have found, as a rule, they are more distinctly mapped out in the 

 brain of the negro than in the white, Plates XLI and XLII, fig. 1. Ecker states 

 that they can best be distinguished in the brain of the new-born child, a fact which 

 I can corroborate. He says that in these brains the typical parts can be recognized 

 almost as in a diagram. I have no doubt that this, as a general statement, is 

 perfectly true ; but there are exceptions, as in the brain of one new-born child that 

 I carefully studied. I found the frontal convolution almost as complex as in the 

 average adult brain. It is difficult, therefore, as Bischoff remarks, to determine to 

 what extent development of the convolutions proceeds after birth. The great indi- 

 vidual variety as regards the development of secondary and tertiary convolutions 

 in the adult brain, together with the fact of the great differences as regards these 

 points in the brain at birth, renders the solution of the question as to the amount 

 and .degree of development almost impossible. 



Gratiolet states that in the European the lower frontal fissure is the most clearly 

 defined, while the upper and middle more often run into each other. In the Bush- 

 man, he says, on the contrary, the upper remains independent, while the middle 

 and lower blend. From the study of a large number of negro brains as obtained 

 in the United States, this has not been my experience. In those examined, the 

 lower frontal was the most clearly separated, while the upper and middle blended 

 most with each other. The distinctness of the inferior frontal fissure is well seen 

 in Plates XLI and XLII, fig. 1. In the mulatto, Plate XLIV, fig. 1, the 

 two fissures were about equally distinct, the lower one being very much arched 

 upward, the lower frontal convolution becoming the largest I have met with. It 

 was about two-thirds the width of the superior and middle combined. This is 

 exceptional, the middle frontal being almost constantly the widest of the three. 

 On the orbital surface of the frontal lobe are found several small sulculi which are 

 somewhat variable in their arrangements in different brains. They generally 

 assume a tri-radiate appearance, or that of an elongated letter H, or like two Y's 

 placed stem to stem, whence the name of the tri-radiate sulcus of Turner, the 

 solco-crnciforme of Rolando. Ecker terms it the sulcus orbitalis, which appears to 

 be the best name. These sulci seem to bear no definite relation to the general 

 morphological type of the hemisphere, but appear to be due chiefly to local con- 

 ditions dependent upon the adaptation of the inferior or orbital surface of the 

 occipito-frontal lobe to the surface of the orbital plates of the frontal bone. 

 Another fissure is found on this surface which is produced by the presence of 

 the olfactory lobe, which lies embedded in it. This is known as the sulcus 

 olfactorius, and defines a small straight ridge lying between the olfactory bulb 

 and the mesial margin of the hemisphere which is known as the gyrus rectus. 



2. POSTERIOR OR PARIETAL DIVISION OF THE OCCIPITO-FRONTAL LOBE. 



On the lateral surface of the so-called parietal lobe, according to the boundaries 

 I have adopted, nearly all writers distinguish at least three convolutions. Pansch, 



