LOBES OF THE TELENCEPHALON 853 



vary greatly. The main gyri of the two hemispheres of the same brain, how- 

 ever, are nearly alike. 



Origin of the gyri.— The g\Ti (and sulci) are the result of processes of unequal growth — 

 folds necessarily resulting from "the surface portion of the hemispheres increasing much more 

 rapidly than the central core. In the early periods of fetal Ufe the surfaces of the hemispheres 

 are quite smooth. In many of the smaller mammals this condition is retained throughout Ufe, 

 but in the larger mammals, including man, as development proceeds the cerebral cortex becornes 

 thro^Ti into folds. The absolute amount of the grey substance of the hemispheres varies with 

 the bulk of the animal, and apparently with its mental capabilities. This is especially true of 

 the cortex, for in the larger brains, and that of man especially, by far the greater amount of the 

 cerebral grey substance lies on the surface. Therefore, in either tlie growth or evolution of a 

 small animal into a large one the amount of cerebral grey substance is increased, and in this 

 increase the surface area of the brain is necessarily enlarged. It is a geometrical law that in the 

 growth of a body the surface increases with the square, while the volume increases with the 

 cube of the diameter. The cerebral hemisphere is a mass the increase of whose volume does not 

 keep the required pace with the increase of its surface area or cortical layer. The white sub- 

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

 cortical 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 accordance with an 

 increased bulk of body, is greater than is necessary to cover the surface of the geometrical figure 

 formed by the combined white and grey substance. Consequently, in order to possess the 

 preponderant amoimt of grey substance, the surface of the hemisphere is of necessity throwTi 

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

 the hemisphere, the greater and more folded wiU 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 amoimt of their intelligence, while large animals have 

 convoluted brains. 



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

 the fissures, appearing first and in a more or less regular order. Up to the fifth month the en- 

 cephalon, due to its rapid growth, closely 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 the space 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 unre- 

 stricted 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 hmited by a cranial cavity of small size. It is probable that the second contact of the 

 cortex with the cranium (during the eighth month) may at least cause a deepening and accentua- 

 tion of the gyri already begun. Evidently the form of the cranium modifies the gyri, and to a 

 certain extent probably determines their direction, for in long, dolichocephalic crania the an- 

 tero-posterior gyri are most accentuated, and in the wide, brachycephalic crania the transverse 

 gjTi are most marked. At birth aU 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 geometrical 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 satis- 

 factorily 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 of a region varying with the functional impor- 

 tance, and the consequent local increases being accompanied by resultant local foldings. This 

 idea is supported by the fact that while the somsesthetic (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. Of 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 waUs of the sulci and fissures. 



Lobes of the Telencephalon and the Gyri 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 

 use of imaginary lines (figs. 672, 673). These divisions are six in number, them- 

 selves subdivided into their component gyri: — 



(1) Temporal lobe. 



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



(3) Frontal lobe. 



(4) Parietal lobe. 



(5) Occipital lobe. 



