96 OSBORN. [Vou. II. 
EXPLANATION OF PLATE VI. 
The figures, with the exception of Fig. 30, are composites of three or four sections, 
and thus present a larger number of fibres than are in view in any single plane. 
Fics. 24 AND 25. Sections of Cryftobranchus. The former is through the outer 
regions of the encephalic segments; the latter is just external to the mass of central 
gray matter. 
Fic. 25. Transverse section of the mesencephalon of Vecturus at the exit of the 
3d pair of nerves, 25a. An enlarged actual section of the roof showing the connec- 
tion of the descending trigeminal fibres with the cells of the optic tract. Camera 
drawing. 
Fic. 26. Sagittal section through the outer portion of the left optic lobe of Rana. 
Eight layers may be distinguished as follows: 1, the outer fibre layer which contains 
the superior portion of the optic tract; 2, the layer of scattered nerve cells; 3, the 
second fibre layer penetrated by the inferior portion of the optic tract; 4, the second 
layer of compact nerve cells; 5, the third fibre layer; 6, the third fine layer of com- 
pact nerve cells; 7, the fourth fibre layer; 8, the ependyma. The outer fibre layer 
is mostly homogeneous. The second and third fibre layers are traversed by radiating 
fibres which connect the three cell-layers. 
Fics. 27 TO 29. Outer, median, and inner sagittal sections of the brain of Rana. 
The latter is slightly external to the central gray substance. 
Fic. 30. Sagittal section of the brain of Amphiuma. The dotted lines represent 
the actual contours in the lateral plane of this section. The plain lines represent 
the contours of the median plane. 
Fic. 31. A diagram of the main encephalic tracts and nuclei as observed in the 
Urodele Amphibia, a composite of the figures upon Plates V. and VI. The distribu- 
tion of the tracts is a matter of actual observation. The sensory or motor character 
of the tractsis in a measure inferential. .S, main sensory tract. JZ, main motor 
tract. 
