HERNIA. 387 



the egress of certain organs out of the abdomen, either during 

 foetal life or after birth. 



The hernia, when not arising from a traumatic cause, is the 

 result of some violent muscular effort on the part of the animal, 

 in the course of which the viscera are made to exert a violent out- 

 ward pressure upon the walls of the abdomen. If the pressure 

 bears against any of the more soHd portions of the wall, there will 

 be no yielding, and the parts vrill remain uninjured and intact. 

 But if the pressure becomes unduly violent, and the attack is di- 

 rected against some one of the weaker supports, there must be a 

 yielding, and the intestine or omentum, as the case may be, wiU 

 be forced through the openiag which falls most nearly in Une with 

 the direction of the violence. The resistance fails, the viscera 

 passes through the aperture, and there is a protrusion, a rup- 

 ture, a hernia. 



Two elements must enter into the composition of aU hernias, 

 with the exception of eventrations. They are the sac, and the 

 displaced organs which form its contents, and the mode of its 

 formation may be readily comprehended by a consideration of the 

 figures 373, 874, 875, which show the progressive displacement of 

 the peritoneum, as it is pushed through the opening of the ab- 

 domiual walls, by pressure of the intestines, which are also gradu- 

 ally passing through the same opening. The figures show ia what 

 manner the hernial or peritoneal sac is formed. 



The sac ia thus shown to be the prolongation of the perito- 

 neum, which is displaced, moved and distended, or may have sus- 

 tained partial laceration of its fibres. It may even happen that 

 the rupture of that serous membrane is complete, and that it has 

 become lodged in the cellular tissue. In such a case the lacerated 

 peritoneum soon throws out a provisional reparatory sheath, which 

 becomes contiuuous with the natural serous membrane. 



Whether formed by the peritoneum itself, or by a membrane of 

 secondary formation, the sac is always composed of a middle por- 

 tion, or body, and an opening, or ring, which constitutes a means 

 of communication with the abdominal cavity, with a neck or canal, 

 a narrowed portion, uniting together the body and the opening of 

 the tumor. 



But little uniformity exists in the formation of the ring or 

 opening of a hernia. Li some instances it is round or oblong, in 

 others it is a narrow slit, and again it exhibits a triangular outline. 



