No. 1.] SPINAL NERVES OF THE DOMESTIC PIGEON. 131 
change in the former. But cases are often found in dissection 
which show that the supply of spinal nerves is not always con- 
stant in this relation to the number of vertebrz, so that nerves 
may appear in the next space behind, in one of the two that are 
often, if not ordinarily, left vacant. Thus Fig. 5 represents 
such a dissection where a new term %, is introduced into s, of a 
series of six free caudal vertebre. A comparison with Fig. 2 
will show that the nerve supply has undergone less reduction by _ 
one pair of spinal nerves proceeding into s,; or that the nerve 
supply is here better developed than in other cases. The same 
variation was met with in dissections of specimens with seven 
free caudal vertebrz, so that another figure might be drawn in 
all respects similar to Fig. 3 with the exception that an x, would 
appear in s; (cf. Fig. 3). This was made out in a large white 
male fan with seven actually free vertebrz, and no less distinctly 
in a common dull-colored male bird of the same number. The 
other cases of seven vertebrze that were dissected proved to be 
as represented in Fig. 3. This occurrence of the extra pair of 
nerves is of course merely the fulfilling of ordinary conditions 
of the spinal column, which here may or may not take place, 
just as the last vertebrze in the series may or may not be articu- 
lated with the coccyx, instead of being ossified with it. And 
there is no reason to suppose that the very last space might not 
in some cases be furnished with nerves also, although there is no 
evidence of this occurring in the many specimens examined. 
Sections of the vertebral column and the cord in place show 
the arrangement of the investing membranes, the relative size 
and structure of the filum terminale, the lumen of the cord, and 
the exit of the nerves. The dura mater investment extends to 
the extremity of the vertebral canal, which runs at least three- 
quarters of the length of the coccyx. This membrane contains 
blood-vessels running in it, one of which passes up into the pia 
mater, in which it may be traced far forward ; it is in connection 
with this that the pia mater and the end of the filum first appear, 
some twenty sections anterior to the end of the spinal canal. 
The lumen of the cord itself first appears in the filum within five 
or six sections of the end. Thus the relations of the pia mater, 
the blood-vessel, and the lumen of the cord are shown in Fig. 6; 
—the section being much magnified, so that the space between 
the dura mater and the pia mater is not brought into the field. 
