434 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



thalamus, and reach the posterior commissure ; but this course, 

 on account of the curves which it describes inwards, is not seen 

 in section-specimens, though it may be easily demonstrated by 

 separating the fibres. The fasciculi alternate very regularly 

 with layers of grey matter, of equal thickness with themselves, 

 in the cells of which, elongated in the same direction as the 

 fibres, and having a long diameter of 30 JJL, and a breadth of 

 10 JJL, they terminate as fasciculi of the cerebral hemispheres, 

 and arise as fasciculi of the crus cerebri. For after the fasci- 

 culi of the posterior commissure have extended about five 

 millimeters beyond the median line, they enter the anterior 

 part of the tegmentum of the crus (fig. 247, Ch), within which 

 they descend as fasciculi of origin of the spinal cord. 



The posterior and external layer of the optic thalamus pre- 

 sents a superficial resemblance to the internal structure of the 

 lenticular nucleus, as in both the medulla of the hemispheres, 

 extending radially to its terminal cells, is crossed by concen- 

 trically arranged medullary lamellae, which are roots of origin 

 of the crus cerebri that arise from these ganglia. Of these con- 

 centric lamellse in the interior of the thalamus (fig. 248, T\ 

 only the innermost, named lamina medullaris by Burdach, 

 and forming the boundary between the imaginary external and 

 internal nucleus of the optic thalamus, has been noticed by 

 authors. But, inasmuch as the lamina medullaris is confined 

 (almost exactly) to the posterior half of the optic thalamus, the 

 separation of these nuclei ceases at all events in the anterior half 

 of the optic thalamus. Luys remarks, on the other hand, that 

 this layer of medulla invests in the centre of the optic thala- 

 mus a nucleus which is sharply defined posteriorly superiorly, 

 below and to the outer side the " Centre median" (fig. 248, (7m). 

 The cells of which this centre is composed do not differ in form 

 and size from those of the remainder of the optic thalamus. Its 

 radiations, belonging to the corona radiata, proceed from the 

 anterior peduncle of the optic thalamus. The lamina medul- 

 laris, however, which bounds the internal nucleus of Burdach, 

 and the " Centre median" of Luys, is not really an independent 

 structure, but only the innermost of a number of concentric 

 medullary laminae, through which, in opposition to the crossed 

 tegmental origin in the posterior commissure, direct roots of 



