426 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



is not sufficiently accurate and precise. The essential form of 

 the optic thalamus is (with various modifications of the di- 

 vision of its mass) that of an arch moulded around a transverse 

 axis, which is the fundamental form also of the lobes of the 

 cerebrum as a whole, and is like that which is characteristic 

 of the nucleus caudatus amongst the ganglionic masses. Just as 

 the lenticular nucleus forms the axis of revolution for the latter 

 arch, so do the transverse ganglionic masses form the axes of 

 revolution for the optic thalamus, partly directly, partly by 

 their transverse processes extending into the medulla of the 

 hemispheres. These axes of revolution of the thalamus are 

 the brachia of the superior and inferior corpora quadrigemina 

 (fig. 243, Q, Bs, Bi), as well as the internal corpora geniculata 

 moulded around the inferior brachia of the corpora quadrige- 

 mina (fig. 243, Gi). The mass of the optic thalamus developed 

 in front of the axis of revolution (fig. 243, Th) is the longest, 

 whilst the portion lying behind this axis, the so-called pul- 

 vinar, or cushion (fig. 243, P), is by far the shortest. 



The thalamus opticus attains its greatest breadth in the 

 pulvinar, and its greatest vertical diameter in front of the axis 

 of revolution, whilst both dimensions are smallest at the an- 

 terior extremity. 



There is only a partial justification for admitting the 

 existence within this general form of special nuclei in the in- 

 terior of the optic thalami ; for the whole of the grey matter 

 of the optic thalamus forms a continuous mass, and no cha- 

 racteristic differences in its textural composition have hitherto 

 been satisfactorily demonstrated. The presence of more or less 

 completely detached nuclei in the optic thalamus is merely de- 

 pendent upon the mode of radiation of the upper links of the 

 projection system, as well as, on the other hand, upon the mode 

 of origin of the cms cerebri in the optic thalamus ; so that 

 with the knowledge of the two latter the knowledge of the 

 former is necessarily associated. 



The upper link of the projection system proceeds from the 

 frontal lobes, from the walls of the Sylvian fissure, and from 

 the temporal lobes to the anterior extremity of the optic tha- 

 lamus, in four forms of radiation. In three directions the 

 substance of the optic thalamus forms obtuse prolongations 



