092 



THE ORGANS OF DIGESTIOX 



Although the thoracic space might be ahnost obliterated, the lung would i^till 

 remain eiitirelv outride the cavity of the serous sac. It would be convenient then 

 to speak of tlie undisturlied layer which still lined the tiioracic space as ihe parietal 

 layer, and the layer which had come in a passive way to invest the lung as the 

 vi'<ceral layer. 



In the abdomen a similar condition exists. Imagine all the viscera to have 

 vanished from the abdominal cavity. A great space would exist, bounded above by 

 the diaphragm, l>elow by the pelvic floor, and in front and behind by the abdominal 

 parietes. This space would be evenly lined by the peritoneum in the form of a 

 simple closed sac. As the viscera reappeared they would emerge, as it were, from 

 the posterior wall of the belly, and would project into the cavity of the abdomen. 

 In so doing thev would push before them the peritoneum lining the posterior wall 

 of the cavitv. aiul would each in turn become invested by the displaced memltrane. 

 All the abdominal viscera are outside the peritoneal cavity, although they are 

 obviously within the abdominal space. The peritoneal sac remains entirely empty, 

 anil its wall, with the exception of the Fallopian orifices, is unbroken. 



That part of the simple sac which lines the anterior abdominal parietes is 

 practically the only part which retains its original connections undisturbed. 



One main function of a serous membrane is to minimise the effects of friction. 

 Those ^^scera wliich are exposed to most movement, such as the small intestines, 



Fig. 592. — Diagram of the Peimitive Alimextary Canal. 

 (A : a indicates the vitello-intestinal duct : b points to the future position of the cseoum.) 



are the most completely invested by the smooth membrane; those whieh ivmain 

 fixed, such as the kidney, are only casually invested. The movements of the 

 viscera in respiration, the alteration in shape and position which Avill occur in such 

 organs as the stomach and bladder, are provided for by the peritoneum in such a 

 manner that these movements are practically without friction. 



The viscera contained in the abdomen are not only numerous, but they are of 

 very complex outline for the most part, and they are disposed in a manner which 

 tends to greatly confuse their relations to one another. 



A bare description of the peritoneum, as it is met with in the human subject, 

 must needs be intricate and complicated, and if considered without reference to 

 more primitive and simpler conditions is almost inexplicable. The descrij^ion 

 may be followed, but it needs interpretation, and no account of this membrane can 

 be "considered to be satisfactory unless it be rendered evident how the complex 

 arrangement has come aljout, and unless the student base his conceptions upon the 

 simple and rudimentary disposition which an elaborate development of parts ami 

 a remarkable specialisation of structure have rendered intricate and confusing. 



The readiest idea of the disposition of the peritoneum is to be derived from a 

 study of the development and most rudimentary forms of that membrane. Such 

 a study is founded upon an examination of the human embryo at various periods, 

 and of the peritoneum in the mammalia and lower animals. 



