24 Anatomy of the Rabbit. 



especially to a position between medial and lateral, but this restriction 

 is perhaps not justifiable. 



In considering the extent of bilateral symmetry, it is necessary to bear 

 in mind that, although a fundamental feature in vertebrates, it is not 

 perfectly retained in the adult condition. Symmetry is destroyed by the 

 migration of an unpaired structure from a median to a lateral position, 

 as is seen, for example, in the case of the abdominal portion of the ali- 

 mentary canal; or, again, by the reduction or disappearance of structures 

 belonging to one side of the body, as, for example, in the case of the 

 mammalian aortic arch. 



Referring to centre and circumference, either in the body as a whole, 

 or in particular parts, the terms deep and superficial, central and periph- 

 eral, or internal and external may be applied. It may be observed, 

 however, that the terms internal and external are sometimes used in the 

 sense of medial and lateral, both in descriptive language and in the 

 names of parts. 



In comparison with the terms medial (medialis) and median (medi- 

 anus) the term middle (medius) may be used to designate the position of 

 a structure lying between two others, the latter being otherwise desig- 

 nated, for example, as anterior and posterior, or one in the median plane. 



The limbs being more or less independent structures, it is proper to 

 apply to them certain terms otherwise applicable to the main portion of 

 the body. The chief terms not properly applied elsewhere are proximal, 

 meaning nearer the centre or base of attachment, and distal, toward the 

 extremity. In the middle segment of the fore limb the respective positions 

 of the bones (radius and ulna) are indicated as radial and ulnar. The 

 terms tibial and fibular are also applicable, although with less reason, to 

 the corresponding segment of the hind limb. The dorsal and ventral 

 surfaces of the fore foot are described respectively as dorsal and volar, 

 those of the hind foot as dorsal and plantar. 



In determining the identity of structures in a quadrupedal mammal, 

 considerable difficulty may at first arise from the fact that descriptive 

 terms, such as those just defined, are frequently included in the names 

 of parts, the latter being, at the same time, terms applied in the first 

 instance to the human body, in which the recognized relations are some- 

 what different. In comparison with that of a quadrupedal vertebrate 

 the human body occupies a vertical or erect position, and is to be con- 

 sidered as having been rotated upward through ninety degrees on the 

 posterior limbs. The latter accordingly occupy for the most part their 

 original position, and the human arm indeed largely reassumes this 

 position when allowed to hang freely at the side of the body. As in all 

 cases, the face retains its forward direction. Thus the terms anterior 

 and posterior as used in human anatomy mean dorsal and ventral, 

 provided they refer to parts of the body, such as the entire trunk region, 

 which have been affected by rotation. The terms superior and inferior 

 as applied to man are similarly anterior and posterior as applied to a 

 lower form. Since it is unwise to change the form'of the official terms 

 of human anatomy, it becomes necessary to interpret all such terms when 

 used for a quadrupedal mammal according to the relations exhibited by 



