426 THE HORSE. 
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. 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 peritoneum is detached from the upper walls of the 
abdomen, and its layers are separated, as at EE, the viscera cannot be 
reached without dividing it twice ; first, as it lines the walls of the abdo- 
men; and secondly, as it closely covers the organ which it is desired to 
Fic. 2.—SrcTIoNAL PLAN OF THE HORSE’s ABDOMEN BEHIND THE STOMACH AND Liver. 
A. A. Large intestines. F. F. Folds connecting the small intestines 
8 B. B. B. Small intestines. with the same, and called the mesen- 
C. C. C. Peritoneum covering intestines. tery. 
D. D. Peritoneum lining the walls of the ab- Abdominal muscles. 
domen. 
E. E. Folds connecting the large intestine 
with the parietal peritoneum, called 
mesocolon. 
Lumbar muscles. 
G. 
. H. Ribs. 
i 
. J. Kidneys imbedded in fat 
areas") 
open. In certain conditions it is important to remember this, as for 
instance in distended states of the colon, when it may be a questiqn 
whether the bowel may be punctured from the flank without woundins 
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 in health to lubricate the 
surface, but in disease becoming very profuse and greatly altered in 
character. 
THE MUSCULAR COAT OF THE HOLLOW VISCERA varies in thickness; but 
the whole of it belongs to the unstriped division of muscles, and its 
action is purely involuntary. In all but the large intestines the fibres 
are arranged in a circular direction, but in these they are divided into 
