208 COPE. [ VoL. III. 
ball-and-socket, and admits at the same time of more abrupt 
flexure than the latter. It may have arisen in birds from a 
combination of two causes. One of these is the much greater 
frequency of the vertical and 
lateral flexures of the cervical 
region than any others. These 
are seen in the vertical flexure 
of the neck when in ordinary 
repose ; and the horizontal flex- 
ure when the head is placed 
under the wing during sleep. A 
greater tension of the interver- 
tebral ligaments than is found 
in cold-blooded or even other 
warm-blooded invertebrates, 
would encourage an especial ac- 
tivity of nutrition in the regions 
of greatest stress, when the col- 
umn was flexed in the two habit- 
ual directions, and thus the edges 
of the primitive cups be drawn 
out in the directions in which 
we findthem. This would seem 
to be a reasonable explanation 
of the origin of this type in birds, 
and it may apply to the cervical 
region of the monkeys, which 
display it. They frequently sit 
with the head resting on the 
shoulders and the neck bent 
in an anteroposterior sigmoid. 
Whether they lean the head to 
one side or on the shoulder, dur- 
ing sleep, I have not had the 
opportunity to observe. 
The plane vertebra, like the 
other types, was derived from the biconcave or amphiccelous, 
by ossification of the intervertebral cartilage. This cartilage is 
divided, each half forming the epiphysis of a centrum (Hoff- 
man), and in the Mammalia without forming a ball-and-socket 
Figure 44.— Cervical vertebrae of Cynocephalus, showing saddle-shaped centra, 
