THE STRUCTURE OF THE BRAIN STEM 



375 



masses of grey matter the anterior nucleus, the lateral nucleus (the largest 

 of the three), and the mesial nucleus. Its outer surface is in contact with the 

 layer of nerve fibres formed by the crusta of each crus cerebri as it diverges 

 from its fellow to pass up into the cerebral hemispheres. Into this layer, 

 ' the internal capsule,' fibres proceed from all parts of the thalamus to pass 

 to the cerebral cortex. The anterior extremity of the thalamus, known as the 

 anterior tubercle, forms a marked projection into the lateral ventricle. In 

 front of this, the foramen of Monro leads from the third ventricle into the 

 lateral ventricle. This foramen is bounded anteriorly by a strand of fibres, 

 known as the * anterior pillar of the fornix,' which lies just behind the anterior 



FIG. 190. Transverse section through upper part of mid-brain. 

 Th, thalamus ; bra, brachium superior^'; i cqs, anterior (or superior) corpus 

 quadrigeminum ; cgi, cge, internal and external geniculate bodies ; /, fillet ; s, aque- 

 duct ; pZ, posterior longitudinal bundle; r, raphe ; ///, third nerve ; nlll, its 

 nucleus ; Ipp, posterior perforated space ; an, substantia nigra ; cr, crusta ; II, 

 optic tract ; M, medullary centre of the hemisphere ; nc, nucleus caudatus ; st. 

 stria terminalis. 



commissure and forms a conspicuous feature in the anterior part of the 

 lateral wall of the third ventricle. It passes in the wall down to the corpus 

 mammillare. From the corpus mammillare a well-marked bundle of fibres 

 passes up into the optic thalamus to end round the large cells in the anterior 

 nucleus of the thalamus. The posterior extremity of the thalamus forms 

 a definite prominence, the pulvinar. To the outer and back part of the 

 pulvinar two bodies are developed, known as the geniculate bodies. These 

 may be regarded as special outgrowths of the grey matter of the optic 

 thalamus, one of which, the external geniculate body, is in close connection 

 with the fibres from the optic tracts, while the other, the internal geni- 

 culate body, receives fibres from the lateral fillet ultimately derived from 

 the organ of hearing. In a section through the fore part of the mid-brain 

 (Fig. 190) these two bodies may be seen lying to the outer side of the anterior 

 corpora quadrigemina, so that the fore-brain, to a certain extent, enfolds the 

 anterior part of the mid-brain. Below the thalamus at its back part is the 



