91 6 THE NERVE SYSTEM 



the intervening tentorium another fold of the dura. Further description will 

 be restricted to each cerebral hemisphere. 



Configuration of Each Cerebral Hemisphere. Each cerebral hemisphere 

 presents an outer convex surface (fades convexa cerebri), applied to the correspond- 

 ing half of the cranial vault; a mesal flattened surface (fades medialis cerebri), 

 which lies in a sagittal plane, applied to the corresponding surface of the opposite 

 cerebral hemisphere, with the great longitudinal fissure intervening, and for the 

 most part in contact with the falx cerebri; and a basal or ventral surface, of 

 irregular form, resting frontad upon the floors of the anterior and middle cranial 

 fossae, and caudad upon the tentorium cerebelli. 



Prominent in the lateral and ventral views is the blunt projection of the temporal 

 pole, while at the ventrolateral border, nearer the occipital pole, is a slightly 

 marked indentation usually called the preoccipital notch. The deep vallecular 

 depression between the orbital surface and the temporal pole accommodates the 

 greater wing of the sphenoid. 



More or less distinct borders demarcate the surfaces. The arched dorsimesal 

 border intervenes between the mesal and the convex surfaces; a straight mesorbital 

 border intervenes between the orbital and mesal surfaces of the frontal lobe; a 

 ventrolateral border separates the tentorial surface from the lateral, convex sur- 

 face of the occipital and temporal lobes; while an obtuse border the mesoventral 

 or internal occipital border separates the tentorial from the mesal surfaces. 



Cerebral Fissures and Gyres. The surface of each cerebral hemisphere presents 

 alternating depressions or fissures which demarcate gyral elevations the convolu- 

 tions or gyres. 1 The fissures vary in depth from that of a mere shallow groove 

 to as much as 30 mm., and may attain a length of 15 cm. They are more or less 

 sinuous and ramified. They mark the surface with fairly approximate uniformity, 

 that is, one rarely finds an unfissured surface more than 15 to 20 mm. (f inch to 

 i inch) in width. Numerous functional and mechanical influences must be 

 credited with bringing about the complex foldings of the cerebral surface, prin- 

 cipally (a) resistance of the cranium to the expanding brain or "mechanical 

 packing;" (6) differences of growth rate in different parts of the cortical surface; 

 (c) differences of growth rate of different fibre bundles retarding cortical expansion 

 along the fissure lines and elongating to help in the formation of the gyres. The 

 obvious result, whatever the influences may be, is an expansion of the cerebral 

 cortex to an enormous degree, so that, instead of having a surface area of onlv 

 60,000 sq. mm. (if unconvoluted), the average adult cerebrum has a cortical area 

 of 200,000 sq. mm. or more. Furthermore, the vascular pia, closely investing 

 the surface and dipping into every fissure, is expanded in a like manner, affording 

 an ample and uniform supply of blood for the entire cerebral cortex. 



The cerebral vesicle of the fetal brain presents a smooth surface during the 

 first half of intrauterine life, 2 except for the depressed fossa at the site of the future 

 island of Reil destined to become buried in the depths of the sylvian cleft formed 

 by the apposition of the more energetically growing contiguous parts of the cerebral 

 mantle. Some of the cerebral fissures develop early as infoldings of the com- 

 paratively thin wall of the vesicle, and hence produce corresponding projections 

 into the cerebral cavity; these are termed the total or complete fissures. The 

 remaining fissures are only linear depressions of the surface not involving the entire 

 thickness of the wall the partial or incomplete fissures. The complete fissures and 

 their correlative projections into the cerebral cavity (lateral ventricle) are: 



"Consistent with the use of the English lobe and lobes (for lobus and lobi). the English gyre and gyres are 

 preferable to ffyrus and gyri. The term fissure is here uniformly employed for all anfractuosities of the sur- 

 face, though salcus (pi. sulci) is quite as generally used; sometimes both terms are indiscriminately mixed 

 in other works on this subject. 



2 The so-called transitory fissures of older descriptions may be neglected, since the researches of Retzius, 

 Hochstetter, and Mull have shown these to be in reality artefacts due to postmortem swelling. 



