t440 A MANUAL OF ANATOMY 



The locus perforatus posticus is a perforated lamina of grey 

 matter which forms the floor of the interpeduncular fossa, the 

 openings being for the passage of the postero-mesial central or 

 ganglionic branches of the posterior cerebral arteries. This grey 

 lamina extends between the tegmenta of the crura cerebri, and it is 

 hence known as the pons Tarini. 



Ganglion Interpedunculare. — This is a collection of nerve-cells, 

 situated mesially in the lower part of the grey lamina which con- 

 stitutes the locus perforatus posticus. On either side it receives 

 the fibres of the fasciculus retroflexus, which are derived from the 

 ganglion habenulae. 



Structure of the Cerebral Hemispheres. 



The cerebral hemisphere is composed of grey and white matter. 

 The grey matter is disposed externally, and forms the cerebral 

 cortex. The white matter occupies the interior, and constitutes 

 the medullary centre. 



Cerebral Cortex. — The grey matter forms a continuous covering 

 to the entire hemisphere, dipping into the sulci, so as to cover the 

 opposed surfaces of the gyri, as well as the bottom of the sulci. It 

 is thicker over the superficial surfaces of the gyri than at the bottom 

 of the sulci, and attains its greatest thickness over the upper 

 portions of the precentral and post -central g5^i, whilst it is thinnest 

 over the occipital lobe. 



The cerebral cortex is indistinctly divided into strata by means 

 of layers of a whitish substance. When examined on section it 

 therefore presents a stratified appearance, and is seen to consist of 

 successive grey and white layers, alternating with each other. In 

 most parts of the cerebral cortex there are four superimposed 

 strata; but in certain situations, e.g. over the precentral gyrus, 

 there are as many as six. These strata are as follows, from without 

 inwards : 



1. Superficial white layer (pale and narrow). 



2. Superficial grey layer. 



3. Outer white band of Baillarger. 



4. Middle grey layer. 



5. Inner white band of Baillarger. 



6. Inner or deep grey layer, subjacent to which is the 



white matter of the medullary centre. 



The medullated fibres of the medullary centre pass into the 

 stratified grey cortex in a radiating manner, and within the cortex 

 their course, for the most part, is perpendicular to the superficial 

 surface, and between the component cells of the cortex. 



Minute Structure of the Cerebral Cortex. — The cortex is com- 

 posed of nerve-cells and nerve-fibres. 



Nerve-cells. — These are arranged in four layers, which are as 

 follows from without inwards; (i) the molecular layer; (2) the 



