INTERNAL STRICTIRE OF THE BRAIN STEM 995 



nucleus facialis is therefore, in part at least, a cellular specialization of 

 relatively late appearance. Since it innervates those structures of the head 

 and face involved in emotional expression, its greatest functional capacity 

 occurs in those forms ha\ing the most extensive range of emotion. The 

 facial musculature is also engaged in the acts of swallowing and mastication. 

 Not a few (if xhv primates develop large cheek pouches in which they retain 

 masses of food, awaiting later deglutition. During mastication the vestibule 

 oi the mouth, between the teeth and the cheeks, is compressed in order to 

 bring the food between the grinding surfaces of the teeth. The lips during 

 this process also are drawn together, and thus play an essential role in 

 mastication. During the act of swallowing, the cheek recesses of the vestibule 

 are likewise occluded and the lips compressed, so that the bolus of food is 

 forced backward from the mouth to engage between the pillars of the fauces. 

 This masticatory and deglutitional element, innervated by the facial nerve, 

 represents its primitive and basic function. The innervation of the muscles 

 of expression, for portraying states of emotion, is a newer functional attribute 

 of the facial nucleus. As a direct effect of the expansion in emotional expres- 

 sion, the nucleus facialis reaches its highest organization in man. 



Adaptations i\ Masticatohv Nucleus and Reticular Fornlation. 

 A like process of increasing distinctness applies to the nucleus masticatorius 

 which has an even greater antiquity. Many vicissitudes have affected the 

 sensory portion of the trigeminal ner\e, init its motor division has remained 

 constant. 



Progressive increase in the definition of the oculomotor nuclei, including 

 the nucleus abducentis, the nucleus trochlearis and the nucleus oculomotor- 

 ius, may be observed in passing from the lower primates, through the inter- 

 mediate stages, to the great anthropoids and man. Nor is this tendency 

 confined to the cranial nerve nuclei. It is also conspicuous in the reticular 

 formation of the entire brain stem. The exact degree of progressive dclimita- 



