THE TELENCEPHALON. 1135 



its distinctness, is produced by a fold of dura over the parieto- mastoid suture and 

 above the highest part of the lateral sinus (Cunningham). It is of importance 

 in the topography of the brain, since it is often taken as the lower limit of the 

 parieto-occipital line, establishing the conventional division on the lateral surface of 

 the hemisphere between the parietal and occipital lobes (page 1143). 



The complex modelling of the surface of the cerebral hemispheres, the charac- 

 teristic feature of the human brain, is produced by the presence of irregular eleva- 

 tions, the convolutions or gyri, separated by the intervening furrows, the fissures 

 or sulci. Although presenting many variations in the details of their arrangement, 

 not only in different individuals but even in the hemispheres of the same brain, the 

 convolutions and fissures of every normal human brain are grouped according to z. 

 general and definite plan to which the brain-patterns, whether elaborate or simple, in 

 the main conform. The fissures differ greatly not only as to their depth as observed 

 in the fully formed brain, but also as to their relation with the developing hemi- 

 sphere, a very few, known as the complete fissures, involving the entire thickness 

 of the wall of the cerebral vesicle and in consequence producing corresponding eleva- 

 tions on the internal surface of the ventricular cavities. Of such total sulci the most 

 important permanent ones are : (i) the hippocampal fissure, which produces the pro- 

 jection known as the hippocampus major within the lateral ventricle ; (2) the ante- 

 rior part of the calcarine fissure, which gives rise to the calcar avis ; and (3) the 

 fore-part of \h&. collateral fissure, which is responsible for the variable collateral emi- 

 nence. 'Y\vQ. choroidal and the parieto-occipital fissure are also complete fissures of 

 foetal life, but give rise to invaginations which do not permanently model the ventric- 

 ular walls. The remaining furrows merely impress the surface of the hemispheres 

 and are termed incomplete fissures. Their depth varies, in some cases being only a 

 few millimetres and in others as much as 2.5 cm., with an average of about i cm. 

 The height of the convolutions usually exceeds their width, the latter, in turn, being 

 commonly somewhat greater at the surface than at the bases of the gyri. It is evi- 

 dent, therefore, that the convoluted condition of the hemispheres provides a greatly 

 increased area of cortical gray matter without unduly adding to the bulk of the 

 brain, the extent of the sunken surface being estimated as twice that of the exposed. 

 The larger and longer adjacent convolutions are frequently connected by short 

 ridges, the annectant gyri, which have no place in the typical arrangement. They 

 may cross the bottom of the intervening fissure and ordinarily be entirely hidden 

 from view (gyri profundi); or they may be superficially placed (gyri transitivi) and 

 materially add to the complexity of the surface configuration. 



The cause and origin of the cerebral convolutions are still subjects for discussion. The 

 fact, that at the time the fissures begin to appear, towards the end of the fifth fcetal month, the 

 surface of the young brain is not in close contact with the cranial wall, disproves the assumption 

 that the latter is directly responsible for the production of the fissures and convolutions. It is 

 probable that the immediate cause of the surface modelling must be sought in the unequal 

 growth and consequent localized tension which affect the hemispheres, excessive growth in the 

 longitudinal axis resulting in transverse furrows, and that in the opposite axis producing fissures 

 extending lengthwise. Whether the excessive expansion is caused by increase in the gray or 

 white matter is uncertain, although local augmentation of the cortical gray substance is prob- 

 ably the more important factor. After the beginning of the eighth month, when the growing 

 brain comes into contact with the cranial wall, the convolutions, which before were to a large 

 extent unrestrained and therefore relatively broad and rounded, suffer compression, the results 

 of which are seen in the flattening and closer packing of the gyri and the narrowing and deepen- 

 ing of the intervening fissures. By the end of fcetal life the salient features of the plan of 

 arrangement have been established, although the final details of the brain-pattern are not 

 acquired until sometime after birth. 



The Cerebral Lobes and Interlobar Fissures.— For the purposes of 

 description and topography, the cerebral hemispheres are subdivided into more or 

 less definite tracts, the lobes, by certain sulci, appropriately known as the inter- 

 lobar fissures. With few exceptions, however, the lobes so defined have litde 

 fundamental importance, since their recognition is warranted bv convenience and not 

 by morphological significance, in most cases the conspicuous Hmiting sulci being of 



