82 ESSENTIALS OF PHYSIOLOGY. 



depth at the sixth month of foetal life. The outer cell lamina is the 

 last to appear, and develops slowly after birth. The outer fibre lamina 

 is well developed at birth, and is associated in its further growth with 

 that of the outer cell lamina. The inner fibre lamina is well developed 

 at birth, attaining its adult depth almost at once. 



(2) The inner cell lamina is the first to appear in the evolution of 

 the cerebral cortex, and is well developed in the lower mammalia ; in 

 the mole, for example, it forms the greater part of the depth of the 

 cortex. It is followed by the middle cell lamina, the outer being the last 

 to appear, and attaining a low degree of development in all animals below 

 man. The outer cell lamina, however, increases progressively in depth 

 from the insectivora through the rodents and ungulates to the carnivora. 



(3) The least developed portion of the human cerebral cortex is 

 the grey matter of the hippocampus, in which the only cell laminse 

 represented are the middle and inner. The ascending frontal convolu- 

 tion (motor area) is characterised by the presence of Betz cells in 

 the inner- fibre lamina. The visuo-sensory area, situated in the 

 occipital lobe, is distinguished by an increase in depth of the middle 

 cell lamina. The outer cell lamina is most highly developed in what 

 are known as the association areas. These are three in number : the 

 posterior, occupying the posterior part of the parieto-temporal region ; 

 the middle, in the island of Reil ; and the anterior, which lies in that 

 part of the frontal lobe known as the pre-frontal region. The pre-frontal 

 region is the highest zone of association, and in it the outer cell lamina 

 undergoes the greatest development, varying considerably, however, in 

 different individuals. The outer cell lamina in this region is more or 

 less pronounced according as the mental development is of greater or 

 less degree. It is imperfectly developed in idiots and imbeciles, and 

 its cells are atrophied in cases of dementia. 



On the basis of these facts, J. S. Bolton ascribes different functions 

 to the three cell laminae. The polymorphic layer subserves the in- 

 stinctive activities concerned with obtaining food and shelter, with 

 seeking protection from danger, and with the functions connected with 

 sex. The middle cell lamina is concerned with the reception and 

 transformation of afferent impulses. The outer cell lamina is psychic or 

 associational, and has to do with the mental processes, especially with 

 those included under the terms "voluntary attention " and " inhibition." 



THE TRACTS OF THE CEREBRAL HEMISPHERES. 



The nerve fibres of the white matter of the cerebral hemispheres 

 are either projection fibres or they are associational in character, 

 including under the latter term the commissural fibres which connect 



