60 STRUCTURE OF THE RECTUM. 



their centre, as seep in fig. 140, p. 71 ; and which in many sub- 

 jects especially old men, present at their orifices the appearance 

 of black spots. 



The quantity of mucus discharged from the rectum in certain 

 cases of disease, is sometimes very great. Tne internal coat, in 

 consequence of the contraction of the circular fibres exterior to 

 it, sometimes forms longitudinal folds, which have been called 

 its columns: these often disappear when the intestine is opened 

 lengthways and spread out. By the contraction of the longi- 

 tudinal fibres, the internal coat is often thrown into folds or 

 doublings, that must assume a transverse or circular direction ; 

 they occasionally pass down through the sphincter, and form the 

 prolapsus ani. The rectum is most plentifully supplied with 

 blood-vessels, to be described hereafter ; and it may be observed, 

 that, on the lower part of the internal coat, the veins are particu- 

 larly numerous.* 



The internal coat of the rectum terminates abruptly just 

 within the anus, and is united to a production of the skin, which, 

 like the covering of the lips, is very delicate and vascular, and 

 has an epithelium, or very thin cuticle, spread over it. The 

 levator and sphincter ani muscles, with which the termination of 

 the rectum is invested, are described in the first volume. 



The Absorbents of the Intestines, are commonly denominated 

 Lacteals.-^ They originate on the internal surfaces of these 

 viscera, as has been already described. After passing through 

 the lymphatic glands, which are so numerous on the mesentery, 

 they generally unite and form one of the great trunks which 

 compose the thoracic duct. It is asserted, that some of the 

 absorbent vessels of the lower intestines unite to the lymphatics 

 of the loins. 



The Nerves of the Intestines are principally derived from 

 the intercostals, or great sympathetics. From each of these 



* No intestine in proportion to its size, receives so much blood as the rectum, 

 and the veins at its lower part not unfrequently exist in the form of a plexus. 



f The Lacleals were first observed by Erasistratus and Herophilus, of the 

 school of Alexandria, during the reign of the Ptolemies ; and subsequently by 

 Aselli, of Pavia, in 1622, the knowledge of them having been lost for 1900 

 years. H 



