THE DIENCEPHALON. 



1127 



Then, too, many corticifugal fibres course in the opposite direction as the axones of 

 the cortical cells, and end in relation to the geniculate neurones, thus establishing a 

 double relation between the lateral geniculate body and the occipital cortex. 



The Hypothalamus. — Although, strictly regarded according to its develop- 

 mental relations, the diencephalon claims only the posterior or mammillary part of 

 the hypothalamus, it is desirable to consider at this time the derivations of the entire 

 hypothalamic subdivision of the fore-brain. Under the above heading will be de- 

 scribed, therefore, the structures lying within or forming the floor and the anterior 

 wall of the third ventricle, including the subthalamic region. 



The subthalamic region in its developmental relations stands, as it were, as a 

 link connecting the diencephalon and the mid-brain. The subthalamic region is the 

 upward prolongation of the tegmentum of the cerebral peduncles and occupies, on each 

 side of the mid-line, the triangular area between the thalamus above and the internal 

 capsule and its continuation, the crusta of the peduncle, below (Fig. 974). It is insepa- 



FiG. 975. 



Corpus 

 callosum 



Lateral 

 ventricle 



Velum 

 interposituin 



Pineal body, 

 cut 



Superior 



colliculus 



Sylvian 



aqueduct 



Red nucleus 



Substantia 

 nigra 



Choroid plexus 



Fornix 



Pulvinar 



Lateral geniculate 



body (leader crosses 



cut tail of caudate 



nucleus) 



Median geniculate 



body 



Hippocampus 



Superior cerebellar 

 peduncle 



Pons 



Frontal section of brain passing through posterior poles of thalami, pineal body and brain-stem. 



rably blended with the ventral surface of the thalamus, which thus obliquely overlies 

 the termination of the tegmental or sensory portion of the cerebral stalk. Through 

 this area the important thalamocipetal paths of the fillet and of the superior cerebellar 

 peduncles reach the thalamus, and within it are seen the upper extremities of the 

 chief ganglia of the mid-brain, the substantia nigra and the red nucleus, and a new 

 mass of gray matter, the corpus subthalamicuni. The substantia nigra presents the 

 same characteristics here as in the peduncle, being conspicuously dark and overlying 

 the crustal fibres. As it ascends, it decreases in bulk from within outward until, at 

 the level of the mammillary body, the substantia nigra is no longer recognizable. 

 The connections of the cells within the substantia nigra are imperfectly understood, 

 but it is probable that they receive many fibres from the caudate nucleus and the 

 putamen and, perhaps, also from the frontal cortical areas. From the cells, on the 

 other hand, fibres pass into the tegmentum and into the crusta and thence to 

 lower levels. According to Bechterew, some fibres join the fillet-tract and thus 

 reach the superior quadrigeminal bodies. At first the red nucleus is a very 

 prominent feature in frontal sections of the subthalamic region (Fig. 970), appearing 



