376 



Comparative Anatomy — Its History, Aim, and Method 



mesenteries, the result would be the relations found in "II." Having 

 ascertained the connections, the comparative anatomist asserts that 

 "Y" is a dorsal organ and, being structurally similar to "A," is the 

 homolog of "A." For similar reasons, "X" and "B" are homologous. 



Such rotations commonly occur along the digestive tube. In the 

 early embryo the tube is nearly straight. In later development its great 

 increase in length necessitates bending, which may be accompanied by 

 local rotation on its long axis, with the result that mesenteries and 

 organs suspended in them are more or less displaced from their primary 

 median position. In the dogfish, Squalus acanihias, commonly dissected 

 in laboratory courses in anatomy, the anterior region of the adult 

 intestine has undergone a rotation of nearly 180°. The primarily dorsal 

 pancreas is thereby shifted into a ventral position while the point of 

 attachment of the bile-duct to the intestine, midventral in the embryo, 

 is found well up toward the secondarily dorsal side of the intestine. The 

 bile-duct extends between the intestine and that anterior mass of the 

 liver which lies ventral to the stomach and attached to the ventral 

 body-wall by a short mesentery, the suspensory ligament. This ventral 

 attachment identifies the liver as an organ ventral to the digestive tube, 

 regardless of the positions which may be assumed by the long, slender, 

 and quite unsupported lateral lobes of the liver. Therefore the bile- 

 duct, coming from the ventral liver, is a duct ventral to the digestive 

 tube in spite of the nearly dorsal attachment of the duct to the 

 intestine. 



A glance at a row of skulls including those of shark, tailed amphib- 

 ian, bird, reptile, and man (Fig. 297) would not suggest that there 

 could be any similaiity in plan and arrangement of bones in structures 

 so very unlike in form and general appearance. Careful analysis, how- 



Fig. 297. Skulls of (A) shark, (B) urodele amphibian, (C) bird, (D) reptile 

 and (E) man. (A and B, courtesy, Kingsley: "Comparative Anatomy of Verte- 

 brates"; D, courtesy, Kingsley: '"The Vertebrate Skeleton"; E, courtesy, Neal and 

 Hand: "Chordate Anatomy"; Philadelphia, The Blakiston Company. C, courtesy. 

 Owen: "Comparative Anatomy and Physiology of Vertebrates," London, Long- 

 mans, Green & Co., Ltd.) 



