362 
A. M. REESE 
d 
Fig. 13 A section of the rostral canal in its middle region. 
others. The cross section of the canal of type 2 (figs. 14, 14a), as 
has already been said, is very much larger than that of type 1. 
The most striking feature, perhaps, of both types of canal is the 
cartilaginous wall, which consists of a close-set series of carti- 
laginous or fibrous rings ("osseous supports" Solger (79) calls 
them) united by less dense fibrous tissue. These rings serve to 
prevent the collapse of the canal. The rings are not at right angles 
to the long axis of the canal, but form an angle of 60-70 degrees 
with this axis. At the bottom of the canal the rings are distinct 
from each other, and are separated by areas of less dense fibrous 
tissue of about the same width as the rings. As they pass up 
around the sides of the canal the rings split up into a number of 
small branches. These branches spread out and form a sort of net- 
work at the top of the canal. It is the ends of these branches that 
make the margin of the opening of the canal so irregular, as was 
noted in speaking of the superficial appearance of the canals of 
type 1. Thebranchesof the adjacent rings meet, but apparently 
do not fuse with each other; they spread out, to some extent, in a 
vertical as well as in a horizontal direction. The arrangement 
of the rings is shown diagrammatically in fig. 15. The inclination 
of the rings from the vertical is seen best in longitudinal sections. 
If entire pieces of the canal be viewed by transmitted light, 
- under low magnification, the rings appear as a series of dense 
