66 ANATOMY OF THE RABBIT. 
man the hand may be rotated to a supine position in which the 
radius is wholly lateral, or in which the two bones are parallel. 
In the posterior limb (C) all three segments are rotated forward. 
Consequently the bones of the middle segment retain their original 
parallel position with reference to one another. The extension 
angles of the knee and ankle are anterior, the flexion angles posterior. 
They differ from the corresponding angles of the anterior limb 
only at the middle joint, but here the difference amounts to two right 
angles. It will be observed also that the distal or ankle joint of 
the posterior limb retains a primitive condition, in which the foot 
is placed approximately at right angles to the leg; in other words, 
it is of a plantigrade type, one in which the sole of the foot rests 
on the ground. Thus the muscles described as flexors in the foot 
pass over the heel. They are functionally flexors of the toes, but 
extensors of the foot as a whole. Those described as extensors lie 
for the most part on the anterior surface of the tibia, and are 
turned from their original course to the dorsal surface of the foot. 
They are functionally extensors of the toes, but flexors of the foot 
as a whole. 
THE NERVOUS SYSTEM. 
There is probably no system of organs in which external form is 
so little suggestive of actual function as is the case with the nervous 
system. This is perhaps less true of its peripheral portion, con- 
sisting of nerves which can be seen ramifying through all parts of 
the body, than of the central portion comprising the brain and 
spinal cord. In examining the external form as a preliminary 
step to the study of the functional arrangements, it is advisable 
to bear in mind that the nervous system is a great correlating 
mechanism, consisting of central cellular portions, and of con- 
ducting paths to and from these centres to outlying parts of the 
body. 
In accordance with its prime importance and at the same time 
the non-resistant character of the tissue of which it is composed, 
the central nervous system is protected within the canal of the 
vertebral column and cavity of the brain cases. It is furthermore 
surrounded by connective tissue membranes of which one, the 
