THE ABDOMEN AND ITS CONTENTS. 279 



walls approach very closely to the upper. The anterior boundary, 

 as before remarked, is the diaphragm, the plane of which moves 

 considerably in active respiration, causing the flanks, or postero- 

 lateral walls of the abdomen, to rise and flill, in a corresponding 

 manner, and thus to indicate the extent of distress in an exhausted 

 animal, or any peculiarity of breathing, as in " broken wind,'' or 

 in the several inflammatory conditions of the lungs. Posteriorly, 

 the boundary is an open one, being the anterior boundary of the 

 pelvis, and corresponding with the brim of that cavity. Superiorly 

 are the crura of the diaphragm, the lumbar vertebrae, and psoas 

 and iliacus muscles ; and laterally, as well as inferiorly, the ab- 

 dominal muscles, and cartilages of the false ribs. Although the 

 abdominal muscles are capable of great dilatation, yet in the 

 natural condition they maintain a gentle curve only from their 

 pelvic to their costal attachments, and hence the depth and width 

 of the back-ribs and pelvis are the measure of the ordinary 

 capacity of the abdomen. Shallow and narrow back-ribs give a 

 small abdominal cavity, and generally speaking, a correspondingly 

 weak condition of the digestive organs; for though this rule is 

 not invariable, yet it is one which may be held as a sufficient guide 

 for practical purposes. Instances do occur of stout and hearty 

 horses possessed of contracted middle pieces, but they are so rare 

 as to be merely objects of curiosity. The small space which is 

 devoted to the organs of digestion in the horse whose back ribs 

 are shallow will be readily understood by reference to the annexed 

 section, in which the enormous mass of intestines and the liver 

 have been removed, leaving only the stomach and spleen. When 

 the walls of the abdomen are distended laterally and downwards, 

 as they always are in horses at grass, the capacity of the abdomen 

 is at least doubled. 



The contents op the abdomen are the stomach, the liver, 

 the pancreas, the spleen, the small and large intestines, the mesen- 

 teric glands and chyliferous ducts, and the kidneys, together with 

 their vessels and nerves. Some of these organs are fixed close to 

 the spine, as the kidneys and pancreas ; but the others glide upon 

 each other as they are alternately empty or full, and to facilitate 

 this motion they are (like the lungs) invested with a serous coat, 

 the peritoneum. They may be divided into the hollow organs, 

 which form one continuous tube (the alimentary), and the solid 

 viscera, which, with the exception of the spleen, are all of a 

 glandular structure, though difi"ering in their minute anatomy. 

 The alimentary canal consists throughout of three distinct layers : 

 the external serous coat { peritoneal^ ^ the middle or muscular coat, 

 and the internal mucnufi coat, which are united by cellular mem- 

 brane, sometimes regarded as forming two distinct additional coats. 



