No. I.] THE CRANIAL NERVES OF AMPHIBIA. \oj 



results. The term ' co7nponent ' is not necessarily synonymous 

 with 'root,' for often two or more components different in fibre 

 structure, internal origin, distribution, and function are given 

 off as one root, and vice versa, different roots may be composed 

 of similar components. ' Root ' has reference to the number 

 of separate bundles by which a nerve issues from the central 

 nervous system — while by ' component ' we refer to bundles 

 qualitatively different. In some cases, of course, the two are 

 identical. 



A resume of this part treats the nerves collectively as 

 regards their components. 



Second, each of the main components thus determined 

 will be further considered as representing a system, and used 

 as the basis of homologizing the cranial nerves of the Amphibia 

 with those of other orders. 



The chart (PI. XII, A) was reconstructed from a series of 

 transverse sections through the tadpole by plotting out the 

 nerves, etc., upon a sheet of paper ruled in squares, the rela- 

 tion between the thickness of the sections and the magnification 

 having been first ascertained. For conciseness and precision 

 the numbers of the sections are used in the text to indicate 

 distances measured along the longitudinal axis. As the 

 sections were loyu. thick, these numbers divided by lOO will 

 give the actual distances in millimeters and decimals of a milli- 

 meter. Each interval in the scale along the sides of the chart 

 equals lo sections (= lOO/u,), and the numbering corresponds 

 to that used in the text. This correspondence, however, is not 

 always exact, owing to slight changes made in the chart in its 

 preparation. 



I. TJie Trigeminus. 



The Trigemintis emerges from the side of the medulla 

 .10 mm. cephalad of the VII -f VIII roots (866-848). Its exit 

 takes place principally just anterior to the entrance of the 

 posterior branch of the VIII into the auditory capsule. It 

 proceeds obliquely cephalad occupying, together with the other 

 nerves described below, a position in the cranium immediately 

 inside the ventral portion of the auditory capsule. The bulk 



