General Features 23 



nerves, blood-vessels, bone, cartilage, and connective tissues are never 

 directly exposed to external space nor to any of the major internal 

 cavities. The substance of the body-wall is mostly muscle. It is covered 

 externally by skin and internally by peritoneum (Fig. 16). The spe- 

 cialized tissues of visceral organs (e.g., the secretory tissues of liver and 

 pancreas, the excretory tissue of the kidney) are not exposed directly 

 to the coelomic space. Everywhere the peritoneum intervenes. The 

 peritoneum is a coelomic "skin." The muscular wall of the heart is 

 covered externally by the epicardium, which is continuous with the 

 pericardium of the pericardial cavity. In fact, no organ can be said to 

 lie in the coelom except as the peritoneum investing that organ is 

 regarded as a part of the organ. In a strict sense, median abdominal 

 organs lie between the peritoneal sheets of the right and left halves 

 of the body. 



Double-Tubular Structure of Trunk. The trunk region of the 

 vertebrate body may be described as having a double-tubular structure 

 — that is, one tube inside another. The outer tube is the body-wall ; the 

 inner is the digestive tube (Fig. 16). The two tubes are similar as to 

 their gross anatomic structure. The outer tube is mainly muscle, cov- 

 ered by skin outside and lined by peritoneum. The greater part of the 

 thickness of the wall of the inner tube is muscle. It is covered by peri- 

 toneum outside and the tube is lined by a skinlike layer, the digestive 

 (endodermal) epithelium, which, however, is much more than a 

 mere protective covering; it is the essential secreting, digesting, and 

 absorbing layer of the digestive tube. 



