I. GENERAL CONSIDERATIONS ON ANIMAL FORM 



A. DESCRIPTIVE TERMS 



The body of vertebrates is carried in the horizontal position and the various surfaces 

 are designated as follows with reference to this position: 



Dorsal — the back or upper side (posterior in human anatomy) . 



Ventral — the under side (anterior in human anatomy). 



Lateral— the sides, right and left. 



Anterior, cephalic, or cranial — 'the head end of the animal (superior in human anatomy). 



Posterior or caudal — the tail end of the animal (inferior in human anatomy). 



Median — the middle. 



Adverbs made by substituting d for the terminal letter of these words mean "in the direc- 

 tion of, " as craniad, toward the head, caudad, toward the tail, etc. 



Other descriptive terms are: 



Central — 'the part of a system nearest the middle of the animal. 



Peripheral — the part nearest the surface. 



Proximal — near the main mass of the body, as the thigh. 



Distal — away from the main mass of the body, as the toes. 



Superficial — on or near the surface. 



Deep — some distance below the surface. 



Superior — above. 



Inferior — below. 



B. PLANES AND AXES 



The structures of most animals are arranged symmetrically with reference to certain 

 imaginary planes and axes. 



i. The median plane is a vertical longitudinal plane passing from head to tail through the 

 center of the body from dorsal to ventral surfaces. It divides the body into two nearly identi- 

 cal halves, right and left. 



2. The sagittal plane or section is any vertical longitudinal plane through the body — 

 that is, the median plane or any plane parallel to it. Sagittal planes other than the median 

 plane are sometimes designated as parasagittal to avoid misunderstanding. 



3. The horizontal or frontal plane or section is any horizontal longitudinal section through 

 the body — that is, all planes at right angles to the median plane and parallel to the dorsal and 

 ventral surfaces. 



4. The transverse or cross plane or section cuts vertically across the body at right angles 

 to the sagittal and horizontal planes. 



5. The longitudinal or anteroposterior axis is a line in the median sagittal plane extending 

 from head to tail; a sagittal or dorsoventral axis is any line in the median sagittal plane extend- 

 ing from dorsal to ventral surfaces; a transverse or mediolateral axis is any line in the transverse 

 plane running from side to side. 



C. SYMMETRY 



The forms of symmetrical animals are dependent upon the arrangement of their parts 

 with regard to the foregoing axes and planes. There are four fundamental types of animal 

 symmetry — spherical, radial, biradial, and bilateral. Since all vertebrates are bilaterally 



