DESCRIPTIVE TERMS. 



partment, and the great arterial trunk, the aorta, the major part of which occupies 

 the dorsal wall of the space. 



The elongated typical vertebrate body terminates anteriorly in the cephalic 

 segment, posteriorly in the caudal appendage ; between these two poles extends the 

 trunk, from which project the ventrally directed limbs, when these appendages exist. 

 Just as the axial segments, represented by the bodies of the vertebrae, take part, 

 in conjunction with the neural arches, in the formation of the neural canal, so do these 

 segments also aid in forming the supporting framework of the ventral body-cavity 

 in connection with the series of ribs and the sternum. 



Descriptive Terms. — The three chief planes of the vertebrate body are the 

 Sagittal, the transverse, and the frontal. The sagittal plane, when central, passes 

 through the long axis of the body vertically and bisects the ventral or anterior and 

 the dorsal or posterior surfaces. The transverse plane passes through the body 

 at right angles to its long axis and to the sagittal plane. T\\^ frontal plane passes 

 vertically but parallel to the anterior or ventral surface, being at right angles to both 

 the sagittal and transverse planes (Fig. 2.) 



The vertical . position of the long axis in the human body is unique, since man. 



Fig. 2. 



Fig. 3. 



Hh'^ 



Three principal planes of human body. T, T, 

 transverse; S, S, sagittal ; J^, F, frontal. 



Human embryo showing primary relations of 

 limbs, a, a, preaxial surfaces; b, b, postaxial ; 

 .r, J, somitic segments of trunk. 



of all animals, is capable of habitually maintaining the erect posture with full exten- 

 sion of the supporting extremities. The lack of correspondence between the actual 

 position of the chief axis of man and the horizontal fore-and-aft axis of vertebrates 

 in general results in discrepancies when the three principal planes of the human 

 body are compared with those of other animals. Thus, the sagittal plane alone 

 retains the relation, as being at right angles to the plane of the support, in all verte- 

 brates, although in man its greatest expansion is vertical. The transverse plane in 

 man is parallel with the supporting surface, while it is, obviously, at right angles 

 to the corresponding plane in the four-footed vertebrate ; likewise, the frontal plane 

 in man is vertical, while it is horizontal in other animals. 



The various terms employed in describing the actual position of the numerous 

 parts of the human body and their relations to surrounding structures have been 

 adopted with regard to the erect attitude of man and the convenience of the student 

 of human anatomy ; hence, in many cases, they must be recognized as having a 

 hmited specific and technical application and often not directly applicable to other 



