12 Basic Structure of Vertebrates 



The truth is that the vertebral column is only one of several features 

 which identify a vertebrate, and some of the other features are quite as 

 characteristic as a vertebral column. Even if an animal lacks a verte- 

 bral column, its possession of the other characteristics in a strongly 

 marked way constitutes so close a similarity to animals that are liter- 

 ally "vertebrate" as to justify including it in the group with them. 

 The question is, then, what are the several salient features which collec- 

 tively make a vertebrate? 



Symmetry 



The vertebrate is bilaterally symmetric — that is, assuming the 

 animal to be standing on a horizontal substratum, a median longi- 

 tudinal vertical plane divides the animal into right and left equivalent 

 parts each of which is as if it were the mirror image of the other. In 

 describing the form and position of parts of the animal, certain terms 

 are used to designate planes of orientation and linear directions. These 

 terms must be defined because confusion results from use of the same 

 term in reference to a quadruped, the long axis of whose body is ordi- 

 narily in a horizontal position, and to a biped, which assumes an erect 

 attitude. The quadruped walks or runs head foremost. In man that sur- 

 face of the body which is forward during locomotion corresponds to the 

 surface which is beneath in the quadruped. 



The muscles which effect the major movements of the body are at- 

 tached to the vertebral column. Therefore the vertebral column is the 

 mechanical axis of the body. In fishes and tailed amphibians, this 

 axis lies in nearly a straight line. In land vertebrates it is more or less 

 curved in the vertical plane, especially in the region of head and neck 

 (Fig. 3). In the definitions below, the axis is assumed to extend in a 

 straight line and in a horizontal direction. 



The more important terms used to designate position in the body 

 are as follows; 



PLANES 



Sagittal: the median longitudinal vertical plane; the basic plane of 

 bilateral symmetry. Parasagittal is a convenient name for any plane 

 not median, but parallel to the sagittal plane. 



Frontal: any plane cutting the animal parallel to the vertebral 

 axis and perpendicular to the sagittal plane. The term refers primarily 

 to the "front" surface of the human body. 



Transverse: any plane perpendicular to the axis; a "cross section" 

 of the body. 



DIRECTIONS 



Anterior and posterior: primarily direction of locomotion. 



In swimming vertebrates and in quadrupeds, therefore, anterior is 



