General Features 13 



toward the head, posterior toward the tail. In man anterior is toward 

 the "front" surface of the body, posterior is toward the back. Herein 

 lies confusion which may be avoided by using "cephalic"' to denote 

 direction toward the head and "caudal"" to denote that toward the 

 tail, regardless of the animal's locomotor habits. Cephalad and 

 caudad are adverbial forms replacing, respectively, "anteriorly" and 

 "posteriorly." The old terms "anterior" and "posterior" are still 

 commonly used, and safely so when it is clear that the description re- 

 fers to animals other than man. 



Dorsal and ventral: upward and downward, respectively, when 

 the animal's axis is in the horizontal position. 



Right and left: as in man, opposite directions perpendicular to the 

 sagittal plane. 



Proximal and distal : direction toward any axis, base, or center is 

 called "proximal": the reverse direction — that is, toward the surface, 

 periphery, or tip of any part — is called "distal." 



DEPARTURES FROM BILATERAL SYMMETRY 



Perfect symmetry may be nearly or quite realized in the early 

 embryo of a vertebrate. Most organs are either median or paired. A 

 median organ arises exactly in the median plane of the embryo. Paired 

 organs appear symmetrically on either side of that plane. However, in 

 later stages of development, the primary symmetry of organs is more 

 or less modified or, in some of them, may be quite lost. 



The digestive tube is at first median. Later the region of it lying 

 in the body-cavity grows lengthwise faster than the body does. It ac- 

 cordingly becomes more or less bent and coiled (Fig. 6). In the adult 

 the stomach may lie more nearly transversely than lengthwise of the 

 body. The intestine may be many times longer than the cavity in 

 which it lies. The human intestine is about 30 feet long. The abdomi- 

 nal portion of the tube in the adult is usually quite devoid of any sym- 

 metry in its arrangement. The heart, at an early stage, is a simple 

 median tube. Later it becomes bent (Fig. 7) and more or less twisted, 

 and, in the adults of various mammals, lies somewhat to one side. 



The two members of a pair of organs may be unequally developed 

 in the adult. This is true of the female reproductive organs of some 

 fishes. In female birds the reproductive organs of the left side only are 

 functional. Those of the right side are rudimentary or lacking. This 

 condition is evidently connected with the fact that the bird's egg is so 

 large relative to the size of the body. The single long, straight, spike- 

 like tusk of the narwhal (the "unicorn" of the sea) projects forward 

 from the left side of the upper jaw (Fig. 8). It is an exaggerated upper 

 front (incisor) tooth. The corresponding right tooth is rudimentary. 



