476 



THE ANATOMY OF THE HORSE 



glandular structure, though differing in their minute anatomy. The 

 alimentary canal consists thi'oughout of three distinct layers : the external 

 serous coat (peritoneal), the middle or miiscular coat, and the internal 

 mucous coat, which are united by cellular membrane, sometimes regarded 

 as forming two distinct additional coats. 



The peritoneum, like the pleura, is a serous membrane, forming a shut 

 sac, and arranged in such a manner that all the abdominal organs are 

 behind it, and two layers of it must be divided before reaching the interior 

 of any of the organs from the lateral or inferior boundaries of the abdomen. 



Fia. 



A. A, 



C. c. c. 



D. D. 



76.— Sectional Plan of the Horse's Abdomen behind the Stomach and Liver. 



Large intestines. 



Small intestines. 



Peritoneum covering intestines. 



Peritoneum lining the walls of the ab- 

 domen. 



Folds connecting the largo intestine 

 with the iiarietal ijeritoneuni, called 

 mesocolon. 



F. F. Folds connecting the small intestines 

 with the same, and called the mesen- 

 tery. 

 G. Abdominal muscles. 

 H. H. Ribs. 

 1. I. Lumbar muscles. 

 J. J. Kidneys embedded in fat. 



This will be better understood by examining the annexed plan, in which 

 the solid black part represents the interior of the peritoneal sac, a space 

 usually extremely small, but capable of being distended to a great extent 

 by a secretion of serum from the internal surface, as in abdominal dropsy. 

 The white line indicates the whole continuous surface of the peritoneum 

 inclosing the black space, which is exaggerated, in order to render the plan 

 more distinct. It will thus be readily understood that unless the perito- 

 neum is detached from the upper walls of the abdomen, and its layers are 

 separated, as at E E, the viscera cannot be reached without dividing it 

 twice ; first, as it lines the walls of the abdomen ; and secondly, as it closely 

 covers the organ which it is desired to open. In certain conditions it is 

 important to remember this, as for instance in distended states of the 

 colon, when it may be a question whether the bowel may be punctured 

 from the flank without wounding the peritoneum. It is a very delicate 

 and irritable membrane in the horse, and should never be interfered with if 

 it is possible to avoid it. Its secretion is a clear serum, merely sufficient 



