No. 1.] SPINAL NERVES OF THE DOMESTIC PIGEON. 129 
In close relation to the varying vertebrz stand the spinal 
nerves. The dorsal ganglion is very prominent, being smaller 
in size and farther from the cord in the most posterior pairs of 
nerves. Shortly beyond the ganglion the nerve branches, — one 
branch sinking below the lateral process of the vertebra and 
supplying the muscles beneath, the other and smaller running 
backward in the muscles dorsal to the vertebre. In the ordi- 
nary dove-cote pigeon, of six free caudal vertebrae, there would 
be seven intervertebral spaces in which the outgoing spinal 
nerves would lie if they existed (Fig. 2, s:-s;); but on account 
of the mass of connective tissue which invests the coccyx and 
terminal vertebrze, for the attachment of muscles and quills of 
the large tail-feathers, the terminal segments of the vertebral 
column seem to have lost their independent action and to have 
become more or less anchylosed ; the cord within is degenerated 
into a large filum terminale which gives off no nerves posterior 
to the fifth space, Fig. 2, ss. Thus the dissection of the com- 
mon birds, represented in Fig. 2, would ordinarily reveal a series 
of six free caudal vertebrae and seven intervertebral spaces, and 
a continuous series of paired spinal nerves (7-75), leaving the 
neural canal between these vertebrae as far backward as the 
fifth space, s;; leaving the last two spaces without nerves, while 
the cord is continued on far into the coccyx as a large filum 
terminale, gradually diminishing in size until it runs out. The 
nerve roots are seen in the dissected cord to originate some dis- 
tance anterior to the point of exit. A bird having only five free 
caudal vertebrze is comparatively rare, and yet the same relation 
between nerves and vertebrze seems to exist as in the more 
common form with six. Thus the dissection represented in Fig. 
1 shows a series of only five free vertebra, six spaces (S:-Ss), 
and a continuous series of nerves, which stops short, however, 
of the last two spaces as in the former case (Fig. 2), thus show- 
ing a reduction of the cord in accordance with that of the bony 
investment. The filum, as before, is continued far into the 
coccyx. The vertebree in this specimen were all very distinctly 
made out, as there was no imperfect union either withthe sacrum 
or with the coccyx. A variation by the increase of the number 
of free vertebrae to seven is more commonly met with. Darwin 
states that this is the normal constitution of the rock pigeon ;? 
1 Animals and Plants under Domestication, Vol. 1., p. 205. 
