476 TEXT-BOOK OF PHYSIOLOGY. 



A transaction of the thalamus shows that it is not only covered 

 externally but penetrated by white matter, which subdivides its con- 

 tained gray cells into four more or less distinct masses termed nuclei, 

 viz., an anterior, a lateral, occupying the external part of the thalamus, 

 a ventral, close to the entire ventral surface, and a posterior, situated 

 beneath the pulvinar. Beneath and somewhat internal to each 

 optic thalamus there is a region, the subthalamic, consisting of an 

 intricate network of nerve-fibers and several nuclei of gray matter, 

 e. g., the red or tegmental nucleus, the subthalamic nucleus, or Luys' 

 body, and the substantia nigra. 



Though the thalamus has extensive connections with many por- 

 tions of the central nerve system, the most important are with the 

 cortex, the tegmentum, and the optic tracts. 



From the cells of these various nuclei axons emerge which pass into 

 the internal capsule, and through the corona radiata to all portions 

 of the cortex. Those axons which come from the pulvinar and pass 

 to the occipital lobe constitute a part of the optic radiation; those 

 from the lateral and ventral nuclei ultimately reach the parietal lobe ; 

 those from the anterior nucleus pass to the hippocampal and unci- 

 nate convolutions. In a similar manner all portions of the cortex are 

 brought into relation with the thalamus, axons from the cortical cells 

 passing downward to terminate in tufts around the thalamic nuclei. 



The tegmentum is intimately related to the thalamus, though the 

 exact distribution of various strands of fibers is a subject of much 

 discussion. Most of the fibers of the mesial fillet end in tufts around 

 the cells of the ventral and lateral nuclei; other fibers pass directly 

 to the cortex. 



The optic tract sends fibers directly into the pulvinar, around the 

 cells of which they terminate in brush-like expansions. 



SUMMARY OF THE STRUCTURE OF THE MEDULLA, ISTHMUS, 

 AND BASAL GANGLIA. 



Structure of the Central Gray Matter. Though the general 

 arrangement of the central gray matter has been incidentally alluded 

 to in the foregoing presentation of the anatomic features of the 

 medulla and isthmus, it will be convenient to summarize its arrange- 

 ment and structure at this point. 



The gray matter of the cord, of the dorsal aspect of the medulla 

 and pons, of the region surrounding the aqueduct of Sylvius, and of the 

 lining of the third ventricle, constitute practically a continuous system, 

 though presenting modifications in various parts of its extent. In 

 the transition region of the spinal cord and medulla the gray matter 

 of the former becomes much changed in shape owing to the shifting 

 of position of the various tracts of white matter, until in the medulla 



