MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 255 



which the manifestations of intelligence operate, to which is attributed the instincts 

 and the passions, has not a fixed organization and is not as invariable as that of other 

 parts ? Each group of brains has a type which is peculiar to it, and this type is 

 especially manifested by the form of its convolutions." 



Leuret's work was continued by Gratiolet 1 who specially studied the brains of 

 the Simiadae and compared them with those of Man, showing that all the con- 

 volutions found in Man are fully represented in the monkeys only simpler in char- 

 acter. To Gratiolet we are indebted for the nomenclature generally in use 

 at the present day. In naming the convolutions he first described them as found in 

 such genera of the Simiadas as Cebus, Cercopithecus, Cynocephalus etc.; and after- 

 wards applied this nomenclature to the corresponding parts of the human brain. 

 He divides each hemisphere into five lobes : frontal, parietal, occipital, temporo- 

 sphenoidal and central or island of Reil. 



■ As to the posterior boundary of the frontal lobe he is uncertain and does not 

 know whether to place it directly in front of the central fissure or in front of the an- 

 terior central convolution. The parietal lobe lies directly back of the frontal and ex- 

 tends to the occipital lobe, from which it is separated by the so-called fissura perpen- 

 clicularis externa et interna, completely in Man on the mesial surface and incom- 

 pletely on the external or lateral. Below and posteriorly it extends above the 

 upper end of the horizontal branch of the Sylvian fissure and becomes continuous 

 with the convolutions of the temporal lobe. Inferiorly and below, both the frontal 

 and parietal lobes are well defined by the fissura Sjdvii, while on the mesial surface 

 they are continuous. 



The occipital lobe constitutes, according to Gratiolet, the posterior portion of 

 the hemispheres, separated from the parietal lobe in the manner indicated above, while 

 with the temporal lobe it is continuous both on the mesial and lateral surface. In- 

 deed, Gratiolet considered the temporal lobe which lies below the horizontal branch 

 of the fissure of Sylvius and occupies the temporal fossa of the skull, in connection 

 with the lower part of the occipital lobe, under the name of the temporo-sphenoidal 

 lobe. He distinguishes on the lateral surfaces of each of these four lobes, three 

 convolutions together with several so-called lobules. On the mesial surface he dis- 

 tinguishes a convolution surrounding the corpus callosum as the convolution of the 

 internal zone and also a convolution lying below the hippocampal sulcus, as the 

 gyrus hippocampi. Besides these he lays special stress on a number of small con- 

 volutions running between the occipital and the temporal and parietal lobes, which 

 he designates under the name of the pits de passage. They are six in number, 

 four external and two internal ; that is, four on the lateral surface and two on the 

 mesial. We shall speak further on in detail of these divisions of Gratiolet, which 

 are those usually accepted by writers and authorities at the present time. There 

 are many deficiencies in the description as given by this writer, especialh' as re- 

 gards the occipital region, and its application to the brain of Man, to which he gave 



1 " Memoire sur les plis cergbraux de l'Homnie et des Primates. 

 32 JOUEX. A. X. S. PHILA.. VOL. X. 



