262 THE DEVELOPMENT OF THE CHICK 



epithelium, and the optic is really comparable to an intramedul- 

 lary nerve tract, seeing that its termination lies in a part of the 



original wall of the neural tube, viz., tlie retina. 



Groups of medullary neuroblasts giving rise to axones of 



motor cranial nerves are located in the brain as follows, according 



to His: 



Oculo-motor nucleus in the mid-brain. 



Trochlearis nucleus in the isthmus. 



Motor trigeminus nucleus in the zone of the cerebellum, including 

 the descending root. 



Abducens and facialis nuclei, beyond zone of greatest width 

 of the fourth ventricle (auditory sac zone). 



Glossopharyngeus, vagus, in the region of the calamus scrip- 

 tori us. 



Accessorius and hypoglossus, in the region extending to the 

 cervical flexure. 

 These constitute the cranial motor nerve nuclei, and are more 



or less discontinuous. 



The ganglionic nerves or nerve-components of the head arise 



from the following primitive embryonic ganglion-complexes: 



1. Complex of the trigeminus ganglia. 



2. Complex of the acustico-facialis ganglia. 



3. Complex of the glossopharyngeus ganglia. 



4. Complex of the vagus ganglia. 



The early history of these ganglion-complexes has already been 

 considered; they are called complexes because each forms more 

 than one definitive ganglion. It is probable also that each con- 

 tains sympathetic neuroblasts, which may separate out later as dis- 

 tinct ganglia, thus resembling the spinal sympathetic neuroblasts. 



There is no close agreement in the segmentation of the motor 

 neuroblasts within the l^rain and that of the ganglion complexes. 

 For instance, in the region of the trigeminal ganglionic complex, 

 the motor nuclei of the oculo-motor, trochlearis, and trigeminus 

 are found, and in the region of the vagus ganglionic complex, 

 the motor nuclei of vagus, accessorius, and hypoglossus. Thus 

 the medullary and ganglionic nerves of the head are primitively 

 separate by virtue of their separate origins. They may remain 

 entirely so, as in the case of the olfactory, trochbaris, and abdu- 

 cens, or they may unite in the most varied manners to form 

 mixed nerves. 



