MUSEUM OF COMPARATIVE ZOOLOGY. 185 



projects into the fourth ventricle, opposite the external attachment of 

 the nerve. It is therefore with this inner swelling, and not with an 

 outer one, that the fifth nerve is originally connected. It may here be 

 noticed, that neither the third nor fourth cranial nerve arises from the 

 middle of the vesicle with which it is connected, but both are attached 

 to the brain near the line which separates this vesicle from the following. 



Passing from the concavity between the first and second neuromeres 

 to that which separates the second and third, or, in other words, pass- 

 ing from the first medullary ridge (using this term to designate the 

 internal ridges) to the second, we come to the origin of the seventh 

 nerve, anterior to the small neuromere beside which lies the ganglion 

 of the united seventh and eighth nerves. In a series of horizontal 

 sections made at the time when these nerves have just left the neural 

 crest, it will be found that from each side of the third neuromere nerve 

 fibres pass downward and towards the middle of that neuromere, where 

 they unite in a large ganglion. (See Fig. 12b.) This ganglion is ulti- 

 mately and secondarily connected with the convexity of the third neuro- 

 mere. As I remarked above, this neuromere is smaller than any other 

 anterior to the origin of the tenth nerve, consequently the space included 

 between the large roots of the seventh and eighth nerves is small, and 

 might easily be overlooked in transverse or sagittal sections. (See Fig. 

 12 a.) The long axes of the cells composing each nerve are, as usual, 

 parallel to each other, but since the two nerves meet the ganglion from 

 different directions, it follows that the long axes of the cells composing 

 the seventh nerve make an angle with the long axes of those composing 

 the eighth nerve, and the ganglion is consequently connected with the 

 brain by two bands of cells whose long axes diverge (Fig. 12 a.) 



Although the fibres of the ninth nerve lie in close proximity to the 

 fifth neuromere, they may be traced back of the ear capsule to the con- 

 cavity which separates the fourth and fifth neuromeres, and corresponds 

 to the fourth medullary ridge. (See Figs. 11 and 11 a.) The first 

 figure shows the course of the nerve, and passes through the neural 

 crest. The second figure is drawn to show the relation of the nerve 

 to the entire ear capsule and the medullary ridge. They are from the 

 same series. 



Posterior to the ninth nerve, the long commissure of the tenth nerve 

 extends beyond the fourth protovertebra. The peripheral distribution 

 of this nerve, and the extent of its commissure, show it to be com- 

 posed of the fused roots of several spinal nerves. All of the spinal 

 nerves arise opposite the muscle plates (Figs. 13, 13 a) from correspond- 



