Aim and Method of Comparative Anatomy 



375 



similarly located parts of the branchial skeleton of fishes, as did 

 Saint-Hilaire and other early anatomists, in spite of their appreciation 

 of "connections." 



Having observed where an organ is in the animal, the anatomist 

 must next find out where it belongs. Figure 296 represents cross sections 

 in the midtrunk region of two animals of different kinds. In '"I" there 

 is a complete system of mesenteries extending from dorsal to ventral 

 body-wall. An organ, "A," lies in the mesentery dorsal to the intestine, 

 and a different organ, "B," is swung in the ventral mesentery. In the 

 other animal, "II," is found an organ, "X," whose structure resembles 

 that of "B," but "X" is dorsal to the intestine. Ventral to the intestine 

 of "II" is an organ, "Y," which is structurally similar to "A." Struc- 

 tural similarity would indicate that " Y" is the same as "A, " and "X" 

 the same as "B," but "A" and "X" are alike in their dorsal location 

 and "B" and "Y" in their ventral location. However, a study of con - 

 neclions in "II" shows that " Y" lies in a broad sheet of mesentery ex- 

 lending from the middorsal line to the intestine and "X" is supported 

 by a membrane passing indirectly from the intestine to the midventral 

 line. If the "A-i>i/es/me-B" system of "I" were rotated on the long 

 axis of the intestine through an arc of 180°, without changing any 

 attachments and with the consequent necessary expansion of the 



I II 



Fig. 296. The "principle of connections." I and II are cross sections in the 

 trunk region of animals of quite different sorts. Organs A and X are situated dor- 

 sally to the intestine; B and Y are ventral to the intestine. But A and Y are attached 

 by mesentery to the middorsal line of the coelom, while B and X are similarly 

 attached to the midventral line. Structurally A resembles Y and B resembles X. 

 Similarity in their connections, along with similarity in structure, indicates that 

 A corresponds to Y and B to X. 



