300 



NORMAL HISTOLOGY. 



The essential histological elements of the cerebral cortex are the nerve- 

 cells and the nerve-fibres. The importance of the former is evident when 

 their three-fold activity is recalled ( i ) as receptors of afferent or corticipetal 

 impulses, (2) as distributors of the impressions so received to other parts of 

 the brain, and (3) as originators of efferent or corticifugal impulses which 

 control the nuclei from which immediately arise the motor nerves. No sin- 

 gle method of preparation suffices 

 to display satisfactorily both groups 

 of structural elements, for when 

 stains are employed which best 

 bring out the cells, the fibres are in- 

 adequately shown; and, conversely, 

 when methods adapted for the 

 demonstration of the fibres are fol- 

 lowed, the cells are but imperfectly 

 displayed. It is advantageous, 

 therefore, to study the histological 

 details of the brain by more than a 

 single method, combining the re- 

 sults obtained by the use of cellular 

 stains with those yielded by proced- 

 ures exhibiting the fibres. Among 

 the latter, the well-known method 

 of Weigert, or its modifications, 

 has been of great service in extend- 

 ing our knowledge concerning the 

 various fibre-tracts. The methods 

 of silver-impregnation introduced 

 by Golgi, although not producing 

 true staining but only incrustations 

 on the cell and its processes, have 

 materially advanced our knowledge 

 concerning the form of the cell- 

 bodies and the number and extent 

 of the processes of the neurones. 



While varying as to details in 

 different regions, the cerebral cor- 

 tex presents a general plan of struct- 

 ure which may be considered: (a) 

 in relation to the nerve-cells and () 

 in relation to the nerve-fibres. 

 The Nerve-Cells of the Cortex. When sections' cut perpendicular 

 to the surface of the convolution are stained with basic stains (Fig. 348) or 

 prepared after silver impregnation (Fig. 349), the cerebral cortex exhibits 

 four principal layers, which, from without inwards, are: (i) the stratum 

 zonale, (2) the layer of small pyramidal cells, (3) the layer of large pyram- 

 idal cells, and (4) the layer of polymorphic cells. Although each presents 

 characteristics which are distinctive, with the exception of the junction be- 

 tween the first and second layers, where the change is well defined, no sharp 

 demarcation separates the strata, each passing insensibly into the adjoining 

 layer. Neither are the modifications which distinguish the cortex of certain 

 regions abruptly assumed, one type of cortical structure being gradually 

 replaced by another without sudden transition. 



Subpial layer 

 Tangential fibres 



Stratum zonale 



Layer of small 

 pyramidal cells 



Outer stripe of 

 Baillarger 



Layer of large 

 pyramidal cells 



Layer of poly- 

 morphic cells 



Medullary fibres 



FIG. 347. Diagram illustrating composition of cere- 

 bral cortex ; cells in right half, fibres in left half of 

 figure ; A,, large and small pyramidal cells ; C, poly- 

 morphic cells ; D, cell of Martinotti ; , cell of type 

 II; f, association cell; :, i, fibres passing to cortex; 

 2, 2, 2, fibres passing from cortex ; IV, neuroglia cells. 



