94 DAVIDSON BLACK 



it constitutes a retinal area but slightly less specialized than the 

 fovea of birds (Cajal, 9, p. 349). 



Thus it would appear that the intrinsic differentiation of the 

 oculomotor nucleus in reptiles is a reaction in response to the 

 need for a greater nicety of muscular adjustment created by the 

 development in the retina of a point of special visual acuity and 

 is not to be directly correlated with the evolution of a special 

 mechanism of accommodation. 



CONCLUSION 



The cerebral motor nuclear pattern in reptiles is subject to 

 considerable variation among the members of the different orders 

 examined. With regard to the disposition of the visceral motor 

 nuclei, however, one feature in particular has been found in all 

 reptiles examined to be more or less characteristically developed, 

 viz., the common and isolated VII-IX motor nucleus. In no 

 other vertebrate class does this nuclear association become a 

 group character, though it has been observed to occur in such 

 widely dissociated forms as Lophius and Canis. 



It has been suggested above that the central association of the 

 IX motor elements with those of the motor VII and the dissoci- 

 ation of this complex from the motor vagus column is probably 

 to be correlated with the divorce of the glossopharyngeal effectors 

 from primarily respiratory action. Viewed in this light the 

 motor VII-IX. nucleus of reptiles is to be considered as a center 

 chiefly concerned in the deglutition reflex. This would be in 

 accord with Kappers' earlier conception that the reptilian motor 

 VII-IX nuclear association was brought about through the 

 neurobiotactic action of the caudally placed VII-IX taste center. 



In the synapsid and diapsid forms examined the caudal end 

 of the dorsal X motor column is prolonged into the upper seg- 

 ments of the cord to form the nucleus of the accessory nerve. 

 Among the Squamata the accessory nucleus is also well developed 

 in Varanus, but in Boa no indication of the nucleus is to be found. 



Until recently it had been supposed that the absence of the 

 accessory nerve and nucleus in Boa was due to changes subse- 



