998 EVOLUTIONAL MODIFICATIONS 



tion of outline in the lornialio reticularis defies estimation b\ means ol men- 

 suration or other methods of jjrecision. Its comparison in the \arious species 

 ol" the primates con\evs the impression that this cHHuse mass of (jray and 

 wliite matter becomes more clearly defined in ascendmjj; the primate series. 

 This elTect may be the result of increasinp; sharpness in definition ol such 

 nuclei as constitute the origin of the cranial ner\es, or due to the tact that 

 certain important nuclear masses, such as the red nucleus and substantia 

 nigra, gradually emerge from the indiscriminate matrix ol the reticular 

 formation. Such increasing dehnition within the formatio reticularis is a 

 morphological tendency clearly demonstrable in the brain stem. It should 

 not be passed over without due notice of the fact that e\en the diffuse matrix 

 of this structure does not escape the inlluences of that progressive specializa- 

 tion aliecting other ancient parts of the central nervous system. 



Adaptations in the Vestibular Nuclei. Ecjually archaic are those 

 centers w hich recei\e impulses from the jjroprioceptors of the semicircular 

 canals, utricle and saccule. These vestibular luiclei show certain lluctuations 

 which, however, are entirely proportional to the ecjuilibratory needs of 

 particular species. In those forms requiring a most effective l)alancing mecha- 

 nism, the vestibular complex is correspondingly large; while in species less 

 dependent upnn this function, there is an apparent falling off in the general 

 dimensions of the vestibular area. In the main, the receiving stations for 

 this function maintain a minimal standard which is essentially constant 

 throughout the entire order. 



It is also interesting to note that tarsius and mycetes i theSouth American 

 howling monkey) stand at the head of t lu' list as ha\ ing the highest coefficients 

 in the vestibular complex. This superiority in the balancing mechanism is 

 unc]uestionabIy due to the animal's higli degree of s]5eciali/.ation. In mycetes 

 it is incident to development of the prehensile tail. B\ this organ the animal 



