GENERAL STRUCTURE AND COMPOSITION. 3 



physiological characters; and mental traits, since we know 

 that their manifestation depends upon the structural integ- 

 rity of certain organs, are especially phenomena of function 

 and therefore not available for purposes of zoological ar- 

 rangement. 



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

 majority of Mammals go on all lours with the head forward 

 and the back upward, and various apes adopt intermediate 

 positions, confusion is apt to arise in considering correspond- 

 ing parts in man and other animals unless a precise mean- 

 ing be given to such terms as "anterior" and "posterior." 

 Anatomists therefore give those words definite arbitrary sig- 

 nifications. The head end is always an_tej'ior 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 ; rigid and left of course present no difficulty: the 

 terms cephalic and caudal as equivalent, respectively, to ante- 

 rior and posterior, are sometimes used. Moreover, that end 

 of a limb nearer the trunk is spoken of as proximal with refer- 

 ence to the other or distal end. The words upper and loiver 

 may be conveniently used for the relative position of parts in 

 the natural standing position o.f 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, in- 

 sects or oysters, but agrees in many points with the groups of 

 fishes, amphibians, reptiles and birds. These four are there- 

 fore 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. The bony axis is 

 also continued through the neck and extends into the head 

 in a modified form. The ventral cavitf, on the other hand, 

 is confined to the trunk. It contains the main organs con- 



