THK r A7,' /:/;/,'.! /. HEMISPHERES 833 



flic bulk of tin- animal, and apparently xvilli its mental capabilities. 'I hi-s is especially 1 1 uc of 

 the cortex, for in the larder brains, anil thai ol man especially, by tar the greater amount of I he 

 cerebral grey suhstaiiee lies cm I he surface. 'I hercfoie, in either the growth or evulntion of a 

 small animal into a large one the amount of eerelira! grey suhstaiiee is increased, anil in this 

 increase the siirtaee area of the hrain is nece.-arily enlarged. It is a geometrical law that in the 

 growth ol' a body the surface increase- with the square, while the volume increases \vith the 

 cul ic of the diameter. The cerebral hemisphere is a mas.- the increa-e of w I lo-e volume does not 

 keep the re |iiired pare with the increase of il- surface area or cortical layer. 'I 'he white -ub 

 stance which forms the pallium anses in large measure as outgrowths from the cells of the 

 eortieal layer, and thus it can only increase in a certain proportion to the grey substance. 

 Therefore, the surface mantle of grey substance of a hemisphere, enlarged in acccrd.-ince w ith an 

 increased bulk of body, is greater than is nece.-saiy to cover the surface of the geometrieul figure 

 formed by the combined while anil grey substance. ( 'oiise.|Ucntly, in order to posses- the 

 preponderant amount of grey substance, the surface of the hemisphere is of neees-ity thrown 

 into folds. It follows also that the thinner the cortical layer in proportion to the volume of the 

 hemisphere, the greater and more folded will be the surface area. In accordance with this theory 

 small animals have smooth or relatively smooth hemispheres, and that independently of their 

 position in the animal scale or the amount of their intelligence, while large animals have con- 

 voluted brains. 



The sulei in general begin to appear with the fifth month of fetal life, the larger of them, 

 the fissures, appearing fir.-t and in a more or less regular order. Up to the fifth month theenceph- 

 alon. due to its rapid growth, elo-ely occupies the cranial capsule. During the fifth month the 

 cranium begins to grow more rapidly than the encephalon, and a space is formed between the 

 cerebrum and the inner surface of the cranium. This space allows further expansion of the 

 pallium, and at the time it is relatively greatest (during the sixth month) the form and direction 

 of the principal gyri and sulci begin to be indicated. As growth proceeds the unrestricted 

 expansion of the pallium results in the gyri again approaching the wall of the cranium, and during 

 the eighth month of fetal life they again come in contact with it. Finally, the later relative 

 growth of the cranium results in the space found between it and the cortex in the adult. It 

 is obvious that the relation of the cranium may be a factor in the causation of the gyri, for the 

 increase of surface area necessitated by the increased amount of cortical grey substance might 

 be limited by a cranial cavity of small size ami then the increase of surface could only be obtained 

 by folding. It is probable that the second contact of the cortex with the cranium (during the 

 eighth month) may at least cause a deepening and accentuation of the gyri already begun. Evi- 

 dently the form of the cranium modifies the gyri, and to a certain extent probaoly determines 

 their direction, for in long, dolichocephalic crania the antero-posterior gyri are most accentuated, 

 and in the wide, brachycephalio crania the transverse gyri are most marked. At birth all the 

 main fissures and sulci are present, but some of the smaller sulci appear later. In the growing 

 pallium both the bottoms of the sulci as well as the summits of the gyri move away from the geo- 

 metrical center of the hemisphere, the summits more rapidly, and hence the sulci or fissures 

 first formed grow gradually deeper as long as growth continues. 



The mechanical factors in the growth processes which result in the more or less regular 

 arrangement of the gyri of the hemispheres of a given group of animals have not been satisfactorily 

 determined. It has been suggested that the differences in arrangement of the gyri in different 

 groups of animals may be in part dependent upon the functional importance of the various 

 regions the amount of grey substance varying with the functional importance, and the con- 

 sequent local increases being accompanied by resultant local foldings. This idea is supported 

 by the fact that while the somiesthctic (sensory-motor) area of the cortex varies with the bulk 

 of the body, the frontal gyri. so much developed in man and which are one of the chief regions 

 of the associational phenomena, are relatively independent of and do not vary with the weight' 

 of either the body or the brain. 



Surface Area. The total surface area of the adult human telencephalon is about 2300 sq. 

 cm. ( )f this area almost exactly one-third is contained on the outer or exposed surfaces of the 

 gyri, while the other two-thirds is found in the walls of the sulci and fissures. 



LOBES OF THK Ti:i.i;\< I.PHALON AND THE FISSURES AND SULCI 



The folded pallium of each hemisphere is arbitrarily divided into lobes, partly 

 by the use of certain of the main fissures and sulci as boundaries and partly by the 

 uf imaginary lines. These divisions are six in number, themselves subdivided 

 into their component gyri: 



(1) Temporal lobe. 



(2) Central lobe or Insula (Island of Reil). 



(3) Frontal lobe. 



(4) Parietal lobe. 



(5) Occipital lobe. 



(6) Olfactory brain or rhinencephalon (including the structures often grouped 

 under the two names alfucttiri/ Inh, anil liinltir lni ). 



This division of the cortex of the hemisphere is largely a merely topographical 

 one. With the exception of the temporal lobe and the rhinencephalon, it has little 

 of either morphological or functional value. The occipital lobe contains the rec- 

 ognised visual area of the cortex, but this area, as such, does not involve allot' the 



