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 fxmctionally 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 



