THE NERVOUS SYSTEM 263 



The motor nuclei of the ociilo-motor, trochlearis, abdiicens, 

 and hypoglossus nerves lie in the same plane as the motor nuclei 

 of the spinal nerves, i.e., in the line of prolongation of the ventral 

 horns of the gray matter. The motor nuclei of the trigeminus, 

 facialis, glossopharyngeus, vagus, and spinal accessory on the 

 other hand lie at a more dorsal level, and the roots emerge there- 

 fore above the level of origin of the others. It will be noted that 

 these are the nerves of the visceral arches, whereas those cranial 

 nerves that continue the series of spinal ventral roots innervate 

 myotomic muscles, like the latter. Similarly the ganglia of the 

 pharyngeal nerves (V, VII, IX, and X) differ from spinal ganglia 

 in certain important respects: the latter are derived entirely 

 from the neural crest, whereas a certain portion of each of the 

 primary cranial ganglia is derived from the lateral ectoderm of 

 the head, as noted in the preceding chapter. Thus the pharyn- 

 geal nerves form embryologically a class by themselves, both 

 as regards the medullary and also the ganglionic components. 



1. The Olfactory Nerve. The embryonic origin of the olfactory 

 nerve has been a subject of much difference of opinion: thus it 

 has been maintained by a considerable number of workers that 

 it arises from a group of cells on each side situated between the 

 fore-brain and olfactory pits; some of these maintained that 

 these cells arose as an outgrowth from the fore-brain, others 

 that they came from the epithelium of the olfactory pit, and 

 yet others that this group of cells, or olfactory ganglion, was 

 derived from both sources. This group of cells was supposed 

 by some to include a large number of bipolar neuroblasts, one 

 process of which grew towards the olfactory epithelium and 

 the other tow^ards the fore-brain, entering the olfactory lobe 

 and ending there in terminal arborization. This view^ is, however, 

 in conflict with the ascertained fact that the fibers of the fully 

 formed olfactory nerve are centripetal processes of olfactory 

 sensory cells situated in the olfactory epithelium. 



The most satisfactory account of the origin of the olfactory 

 nerve in the chick is that of Disse. This author finds two kinds 

 of cells in the olfactory epithelium of a three-day chick, viz., 

 epithelial cells, and germinal cells which become embryonic 

 nerve-cells or neuroblasts. At this time the olfactory epithelium 

 is separated from the wall of the fore-brain by only a very thin 

 layer of mesenchyme. Early on the fourth day axones arise 



