24 INTRODUCTION. 



40. Dorsal and Ventral. By their derivation, and by their appli- 

 cation to distinctly differentiated and universally recognized regions, these 

 two terms are perfectly acceptable so far as the vertebrates are concerned. 



Should the alleged correspondence of the ventral region of the Vertebrate with the 

 tergal region of the Arthropod prove to be one of true homology, it may be desirable in 

 time to discard dorsal and ventral for more suitable terms, but for the present, if on 

 practical grounds alone, it seems well to retain them. 



As stated above, these are among the terms recommended in Quain, and 

 they have been somewhat generally employed by s?iatomists ; Huxley, A, 1 

 et seq. ; Owen, A, III, 3 ; Sanders, 1 ; Hadlich /, 97, note ; Mivart, A, 69, 

 B, 263 and 1, fere. 



To avoid ambiguity, it will be better to employ dorsal only in the sense 

 here indicated, and to characterize the costiferous vertebras as thoracic. 



41. Dextral and Sinistral. These are simply more technical 

 equivalents of the English right and left. No objection has been made to 

 them, or substitutes offered for them. 



42. Lateral. Usually the two sides are so similar that what is true 

 of the one is, approximately at least, true also of the other. Hence it is 

 often desirable to employ a general term applicable to either or both the right 

 or the left. Such a term is lateral, the derivation of which properly indi- 

 cates the fact that, as compared with the two ends or the top and bottom, 

 the right and the left are the sides. Lateral is therefore a more general 

 term, while dextral and sinistral are specific. 



43. Mesal. In the restricted literal sense of entering into the com- 

 position of the head or the tail, certain parts may be said to be absolutely 

 cephalic or caudal ; but in most cases these terms, like dorsal and ventral, 

 merely indicate a greater proximity of the part so characterized to one or 

 the other of the four aspects in comparison with some other part, named or 

 implied. Even when used in an apparently absolute sense, they are really 

 relative terms. 



This is because there has not been detected any structural line or plane 

 of demarcation between the two ends of the body, or between the back and 

 the belly. 



But with right and left, the case is different. The right eye and the 

 right kidney are not simply nearer than the left to the right side or aspect 

 of the body ; they also form parts of the right half of the body, and this 

 half, notwithstanding the numerous instances of continuity, is so readily 

 distinguishable from the left, that one may almost describe the body as if it 

 were formed by the union of two pieces, similar, but reversed in form and 

 position. 



The plane of junction of the right and left sides is at or near the middle 

 of the body, and has been known as the middle or median plane, a longi- 



