GENERAL STRUCTUKE OF THE ALIMENTARY CANAL. 27 



CHAPTER II. 



OF THE GENERAL STRUCTURE OF THE ALIMENTARY CANAL. 

 OF THE OESOPHAGUS, THE STOMACH, AND THE INTESTINES. 



General Structure of the Alimentary Canal. 



— This canal, formed by the oesophagus, stomach, and intes- 

 tines, extends inclusively from the pharynx to the anus. It is 

 composed throughout, (with the exception of the oesophagus, 

 the transverse part of the duodenum, and the termination of 

 the rectum, which wants the serous coat,) of four membranes or 

 tunics ; an internal mucous coat ; one exterior to this, Jihro- 

 cellular (or nervous as it was formerly called) ; a third more 

 external still, which is muscular; and of a fourth investing serous 

 coat, which is derived from the peritoneum lining the cavity of 

 the abdomen. There enter also into its composition, mucous 

 glands, nerves, blood-vessels, and absorbents. The arrangement 

 of the serous tunic varies in different portions of the canal ; but 

 its general disposition is such, that it embraces the canal, and 

 serves as a ligament to connect it to the back and lateral parts 

 of the abdominal parietes, so as effectually to prevent any entan- 

 glement or knotting in the folds of the canal, and at the same 

 time allow it great latitude of motion. It is united to the outer 

 face of the muscular coat, by cellular tissue. The muscular tunic 

 is of a pale color, soft, and easily lacerated. 

 — The muscular tunic consists throughout of two orders of fibres, 

 a circular which is internal, and a longitudinal which is exter- 

 nal. The former are not entirely circular, but consist of seg- 

 ments, which, by their reunion, constitute perfect circles. The 

 longitudinal do not run the whole length of the tube, but are 

 interrupted after short courses. Both may be considered as 

 having their origin and insertion in the fibro-cellular tunic, 

 which separate them from the mucous membrane. The fibro- 



