32 INTRODUCTION. 



the sake of illustrating some of the more common cases of designation of 

 relative position and lines of direction. 



The points A, B and C lie at or upon the meson, and are therefore mcsal. 

 So also, the dotted lines between the points are mesal lines. 



D is not at the meson, and is therefore laterad (in this case sinistrad) 

 of B ; but it lies nearer the meson than E, and is therefore mesad of it. 



B lies cephalad of C, but caudad of A. 



E lies latero-cephalad of C, and latero-caudad of A. 



A lies meso-cephalad of E, and C meso-caudad. The line A, B, C is a 

 cephalo-caudal, or caudo-cephalic line, or it may be described as extending 

 caudad from A, or cephalad from C. 



The line E, D, B, is a mcso-sinistral, or sinistro-mesal line, and may be 

 said to extend either mesad from E, or laterad (sinistrad) from B. 



The line A, E extends meso-cephalad from E, or sinistro-caiidad from A. 



The line A, B, C coincides with the meson, and a cut upon it would be 

 a hemisection. 



The line E, D, B, E' is a transverse line, or at right angles with the 

 meson, and a cut therein would be a transection. 



Finally, an organ on the meson, and represented, for instance, by B, is 

 not only mesal in position, but also called azygous or unpaired ; while two 

 similar organs, one upon each side, and represented for instance by E', are 

 lateral in position, and called paired organs. Each such paired organ may 

 be called the platetrope of the other, or its lateral Iiomologue, or the fellow 

 of the opposite side. 



The letters upon the right leg have similar relations, excepting that 

 proximal and distal take the place of mesal and lateral or dexiral and 

 sinistral. 



I and K, for example, lie respectively cephalad and caudad of G ; while 

 G lies caudad of I and cephalad of K. F lies proximad of G, and G of H : 

 II is distad of G, and the relative locations of the three may be designated 

 ;is proximal, distal and intermediate. 



A. The reasons for repressnting the limbs in the position here given them will be 

 more advantageously presented in 80. 



B. The dotted lines connecting the two extremities of the soma with the distal ends 

 of the limbs illustrate the idea referred to on p. 26 that the limbs are essentially remnants 

 of two continuous lateral folds ; see also Balfour, A, III, 501. 



58. Figure 3 represents an ideal transection of the body of a cat, or 

 ;my other vertebrate possessing limbs. 



Only some of the more constant and essential features are here shown. 



Near the middle of the figure is the section of the Columna vertebralis. 



The Columna vertebralis or spinal column may be of bone, cartilage, or 

 a semi-solid material with membraneous walls, according to the animal ex- 



