GENERAL STRUCTURE AND COMPOSITION 3 



function and therefore not available for purposes of zoological 

 arrangement. 



As man walks erect' with head upward, while the great majority 

 of Mammals go on all fours with the head forward and the back 

 upward, and various apes adopt intermediate positions, confusion 

 is apt to arise in considering corresponding parts in man and other 

 animals unless a precise meaning be given to such terms as " an- 

 terior" and "posterior." Anatomists, therefore, give those words 

 definite arbitrary significations. The head end is always anterior 

 whatever the natural position of the animal, and the opposite end 

 posterior; the belly side is spoken of as ventral, and the opposite 

 side as dorsal; right and left of course present no difficulty: the 

 terms cephalic and caudal as equivalent, respectively, to anterior 

 and posterior, are sometimes used. Moreover, that end of a limb 

 nearer the trunk is spoken of as proximal with reference to the 

 other or distal end. The words upper and lower may be con- 

 veniently used for the relative position of parts in the natural 

 standing position of the animal. 



The Vertebrate Plan of Structure. Neglecting such merely 

 apparent differences as arise from the differences of normal posture 

 above pointed out, we find that man's own zoological class, the 

 Mammals, differs very widely in its broad structural plan from the 

 groups including sea-anemones, insects or oysters, but agrees in 

 many points with the groups of fishes, amphibians, reptiles and 

 birds. These four are therefore placed with man and all other 

 Mammals in one great division of the animal kingdom known as 

 the Vertebrata. The main anatomical character of all vertebrate 

 animals is the presence in the trunk of the body of two cavities, a 

 dorsal and a ventral, separated by a solid partition ; in the adults of 

 nearly all vertebrate animals, a hard axis, the vertebral column 

 (backbone or spine), develops in this partition and forms a central 

 support for the rest of the Body (Fig. 2, ee) . The dorsal cavity is 

 continued through the neck, when there is one, into the head, and 

 there widens out. Within it are inclosed the chief organs of the 

 nervous system. The bony axis is also continued through the 

 neck and extends into the head in a modified form. The ventral 

 cavity, on the other hand, is confined to the trunk. It contains the 

 main organs connected with the blood-flow together with those of 

 digestion and respiration. 



