48 MORPHOLOGY OF THE VERTEBRATE BODY 



functional relation to the kidneys. The heart lies in its sac-like 

 pericardium in the anterior ventral region of the coelome. The 

 spleen, an organ whose function is not entirely certain, lies 

 within the membrane, or mesentenj, that holds the digestive tract 

 in position, but has no special connection with the tract, as have 

 the liver, pancreas, and urinary bladder. The lungs, which lie 

 right and left in the dorso-lateral region, project into the coelome. 

 Their connection with the digestive tract is through the glottis 

 at the posterior end of the mouth. 



For the complete understanding of these organs, comparisons 

 should be made between the lateral view (Fig. 23) and a diagram- 

 matic transverse section (Fig. 19), together with the ventral 

 view as seen by dissection (Fig. 27). In the schematic section 

 through the region of the kidney and reproductive organs the liver 

 is omitted, and instead of showing the digestive tract cut in several 

 places and at irregular angles, a single transverse section is indi- 

 cated. With these ventral, lateral, and transverse views in mind, 

 one can understand the nature of the peritoneum, which is the shiny 

 membrane covering all the organs of the coelome and lining the 

 coelomic walls as wall paper lines a room. The mesenteries, 

 which are also a part of the peritoneum, and which have the func- 

 tion of holding the digestive tract and other organs in position, 

 appear as delicate sheets of tissue containing nerves and blood 

 vessels. The relationships of the peritoneum in these various 

 regions can be understood by reference to the transverse sec- 

 tion, where the peritoneum is seen on edge and, therefore, 

 appears as a line. If its course is followed in such a section, the 

 parietal peritoneum, which lines the coelomic cavity, is continued 

 dorsally across the ventral faces of the kidneys, save where it 

 passes ventrally as a two-layered membrane, the mesentery of the 

 testis or the ovary. At the mid-dorsal region these right and left 

 layers of parietal peritoneum unite to form the mesentery, which 

 is thus composed of two layers of peritoneum and supports the 

 digestive tract. Upon reaching the digestive tract, the ovary, 

 or the testis, the layers of the mesentery separate and extend around 

 these organs as the visceral peritoneum. Hence, if one begins on 

 the ventral body wall and traces the Une of the peritoneum 

 right or left, the line is continuous, since the peritoneum is an 

 unbroken membrane, lining the coelome, passing ventrally as the 

 mesenteries, and covering each of the coelomic organs. The mesen- 



