Chapter II 



RECONSTRUCTION OF THE GRAY MATTER IN THE 

 BRAIN STEM OF LEMUR MONGOZ 



Y' I ^HE impression conveyed by a survey of cross sections of the brain 

 I stem does not give any such realistic idea of the nuclear masses as 

 H that obtained from a study of reconstructions by the Bourne 

 method. Such reconstructions of the gray matter are here employed to make 

 more comprehensible the proportions of those nuclear structures which bear 

 the most significant evidence of evolutional unfolding in the brain stem. It is 

 to be noted, however, that these reconstructions do not disclose the dmien- 

 sions or rehitions of the nuclear masses which occupy positions within the 

 reticular formation. It is equally notewortliy that these deeper lying celhilar 

 collections constitute the more archaically fixed and least variable elements in 

 the composition of the axial graj' matter, while the superficial nuclear aggre- 

 gations represent the recently acquired, more plastic structures of the stem. 

 The tri-dimensional demonstration of the gray matter afforded by recon- 

 structions facilitates the visualization of these elements and establishes for 

 each structure a discrete mf)rphoIogical entity of its own. 



In the following descriptions, only the structures seeming to have salient 

 evolutional significance have been selected for discussion. Any complete 

 anatomical analysis of the constituents of the brain stem belongs more 

 properly to the realm of an anatomical atlas. The structures to be considered 

 here include: the dorsal sensory nuclei, the mferior olivary nucleus, the reticu- 

 lar formation, the pontile nuclei, the vestibular nuclei, the cochlear nucleus, 

 the inferior and superior colliculi, the substantia nigra and the red nucleus. 



The Dorsal Sensory Nuclei 

 In lemur the sensory nucleus of Goll begins as a slender extension 

 from the central gray column close to the midline and separated from 



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