THE STRUCTURE OF THE BRAIN STEM 425 



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 geniculate 

 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. 191) 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 prolongation forwards of the teg- 

 mentum of the crus. This is often spoken of as the subthalamic 

 region. The red nucleus is a conspicuous object in sections through 

 the back part of this region, but gradually diminishes as we proceed 

 forwards, and disappears before the level of the corpora mammil- 

 laria is reached. The mesial fillet, which in the mid-brain lies on the 

 lateral and dorsal aspect of the red nucleus, is prolonged upwards 

 together with fibres from the superior cerebellar peduncle into the 

 ventral part of the thalamus, where probably all of the fibres end in 

 connection with the thalamic cells. The substantia nigra gradually 

 disappears. Before it has disappeared we may see on its outer side 

 a special collection of grey matter called the nucleus of Luys or the 

 corpus subthalamicum. In addition to the anterior and posterior 

 commissures already described as connecting the two optic thalami 

 at the front and back of the third ventricle, the two sides are con- 

 nected about the middle of the cavity by the middle or soft commis- 

 sure. The optic thalamus is often described together with the corpus 

 striatum as forming the basal ganglia. The corpus striatum is, 

 however, genetically, and probably functionally, part of the cerebral 

 hemispheres, and its connections will therefore be best dealt with 

 when describing the latter bodies. 



THE AXIAL GREY MATTER 



In the spinal cord we could distinguish between the anterior grey 

 matter giving origin to the motor nerves, the posterior grey matter 

 serving as an end station for a number of the sensory posterior root 

 fibres, and a lateral horn, less well marked, probably giving origin 

 to the visceral system of nerves. As the central canal widens out to 

 form the fourth ventricle, the relative position of these various parts 

 becomes altered, the anterior grey matter being now nearest the 

 median line, while the posterior grey matter lies more laterally. Part 

 of the lateral grey matter seems to lie deeper than the rest, from 

 which it is separated by the tangle of fibres and cells known as the 

 formatio reticularis. All the cranial nerves from the third to the 

 twelfth arise or end in the axial grey matter, or in close proximity 



