798 A MANUAL OF ANATOMY 



intestine. The mucous membrane which covers the opposed 

 surfaces of the segments belongs to the ileum, and is, therefore, 

 provided with villi. The mucous membrane of the other surfaces, 

 that is to say, the surfaces which look away from each other (down- 

 wards and upwards respectively), belong to the cacum and colon, 

 and are destitute of villi. The villi thus disappear at the margins 

 of the segments. The valve may be described as being formed by 

 an inversion of the terminal part of the ileum into the large intestine. 

 That part of the ileum, having passed upwards and to the right with 

 a slight inclination backwards, enters the large intestine. As it 

 does so, it leaves behind its serous and longitudinal muscular coats, 

 and takes with it its circular muscular, submucous, and mucous 

 coats, the corresponding coats of the large intestine accompanying 

 it in the inversion. The ileo-csecal valve prevents the regurgitation 

 of the contents of the caecum into the ileum. It is generally 

 believed that the mode of action of the valve is as follows : when 

 the caecum becomes distended the fraena or retinacula of the valve 

 are stretched, and exercise traction upon its segments, which are 

 thereby brought together, very much as an old-fashioned purse is 

 closed by pulling upon the two strings. 



For the structure of the rectum, see Index. 



Development of the Stomach and Intestinal Canal. — The primitive gut, or 

 archenteron, consists of splanchnopleure — that is to say, entoderm covered 

 externally by splanchnic mesoderm — and it extends from the head-fold to 

 the tail-fold, lying on the ventral aspect of the notochord. It is divided into 

 three parts — namely, the fore-gut, mid-gut, and hind-gut. 



The lore-gut gives rise to (i) the tongue and back part of the oral cavity, 



(2) the pharynx, (3) the oesophagus, (4) the stomach, (5) the greater part of 

 the duodenum, and (6) the organs developed as outgrowths from the foregoing 

 parts. 



The mid-gut gives rise to (i) a small part of the duodenum, (2) the jejunum, 



(3) the ileum, and (4) the colon, except a small portion at its lower end. 



The hind-gut gives rise to (i) a small portion of the colon at its lower end, 

 (2) the rectum, and (3) the portion of the anal canal above the anal valves. 

 The anus and the portion of the anal canal below the anal valves are developed 

 from the proctodcBum, which is an invagination of the ectoderm at the caudal 

 end of the trunk to meet the intestinal entoderm . 



The cephalic end of the primitive gut is closed, and is separated from the 

 stomodseum, or primitive mouth (which is developed from an invagination 

 of the ectoderm), by a septum, known as the pharyngeal membrane. This 

 septum consists of two layers, one of which is formed by the ectoderm of the 

 stomodaeum, whilst the other consists of entoderm belonging to the cephalic 

 or anterior end of the primitive gut (pharyngeal portion). 



The primitive gut is also closed near its caudal end by a septum, called 

 the cloacal membrane. This membrane, from its mode of formation, bears a 

 close resemblance to the pharyngeal membrane. It consists of two layers, 

 one of which is formed by the ectoderm of the exterior, whilst the other is 

 formed by the entoderm of the hind-gut. 



The mid-gut remains for some time in free communication with the yolk- 

 sac. This communication is at first wide, but a constriction takes place on a 

 level with the umbiUcal aperture, so that the mid-gut and yolk-sac now com- 

 municate by a narrow neck, called the vitelline, vitello-intestihal, or umbilical 

 duct. * The ventral body-wall being now formed, and the yolk-sac being extra- 

 embryonic, this duct extends inwards, along the umbilical cord, through the 

 umbilical aperture, and, after entering the embryonic coelom or body-cavity, 



