22 THE FUR SEALS OF THE PRIBILOF ISLANDS. 



walls to permit of an easy examination of their depths. In order to obtain the desired 

 results, after photographing the dorsal and ventral surfaces of the entire brain it was 

 cut across and the crura cerebri or mesencephalon and the cerebellum and oblon- 

 gata separated. The cerebrum was then divided by a section along the median line, 

 separating it as nearly as possible into two ecpial halves. 



REMOVAL OF DURA. 



Thefalx showed an interesting development; its frontal portion, especially in the 

 region of the olfactory bulbs, being of considerable depth, then becoming very shallow 

 along the middle of the length of the cerebrum and becoming very deej) again in the 

 intercerebral cleft in the caudal region of the cerebrum. A distinct longitudinal 

 venous sinus as in the human brain is not present; but in place of it is a vein of 

 some size lying to the right of the intercerebral cleft and receiving the contents of 

 the dorsal cerebral veins. In connection witli the weak development of the falx 

 along the middle of its length, there was noticed an interdigitation of the gyres of 

 the mesal surface of the hemicerebrums in this region. Tiiis intimate overlapping 

 of the gyres on the mesal surfaces of the two hemicerebrums is possibly correlated 

 with the deficiency of growth of the falx here, and may serve in a measure to 

 increase the tirmness of the union of this region and prevent any undue strain upon 

 the callosnm, which lies some little distance from the dorsal surface of the cerebrum. 



This interdigitation of the mesal gyres is also present in the slicep, where the 

 falx is also deficiently developed. If the hemicerebrums be divided with a sharp 

 knife without tirst separating the pial adhesion of the gyres, the gyres will be cut. 

 An artifact of this nature has, indeed, been mistaken by one writer, in an article on 

 riioca, for the cut surface of a bundle of fibers dorsal 1o and larger than the callosum, 

 and designated by him as the comraissura suprema. 



The tentorium in CallorhinnH is very strongly developed, a])parently extending the 

 whole depth of the transverse archlike cleft between the cerebrujn and cerebellum. 

 The tough, fibrous tissue of the tentorium is, moreover, very noticeably reeuforced by 

 the ])resence of osseous tissue. Where the falx joins the tentorium there is an exten- 

 sion of tiiis osseous tissue in a vertical direction into the falx — a circumstance which 

 certainly is not common in the mnjoiity of other animals, but has been noted by 

 Turner in Macrorhinus. 



TERSIINOI.OGY. 



With the existing uncertainty relating to the homology of the fissures of the brains 

 of the carnivora and that of the human species, nuich confusion has resulted in the 

 present nomenclatare. Some have made a direct homology, others have proposed a 

 fissural type solely and only for the lower forms, while still others have blended the two, 

 and some have utilized n system of names devised by themselves. On tlie lateral 

 surface of the various fissured brain types there is at least one fissure — the Sylvian — 

 which is quite constantly present and on the mesal surface the hippocampal fissure. 



In the matter of nomenclature no attempt has been made to follow the law of 

 priority; but those fissural names, whether of old or recent date, which seemed most 

 api)ropriate concerning position and relation have been adoi)ted, and, with perhaps 

 but one or two exceptions, no new names have been introduced. It has been the 

 purpose to use an intrinsic terminology and to substitute for the sometimes indefinite 



