DEVELOPMENT OF THE ALIMENTARY SYSTEM. 921 



The mesentery is first formed of two perpendicular folds, attached to the sides of the 

 spinal column. As the intestine undergoes development, a portion of the peritoneal 

 membrane extends in a quadruple fold from the stomach to the colon, to form the great 

 omentum, which covers the small intestine in front. 



As the head undergoes development, a large cavity appears, which is eventually 

 bounded by the arches that are destined to form the different parts of the face. This is 

 the pharynx. It is entirely independent, in its formation, of the intestinal canal, the 

 latter terminating in a blind extremity at the stomach ; and, between the pharynx and 

 the stomach, there is at first no channel of communication. The anterior portion of the 

 pharynx presents, during the sixth week, a large opening, which is afterward partially 

 closed in the formation of the face. The rest of this cavity remains closed until a com- 

 munication is effected with the oesophagus. The oesophagus appears in the form of a 

 tube, which finally opens into the pharynx above and into the stomach below. At this 

 time, there is really no thoracic cavity, the upper part of the stomach is very near the 

 pharynx, the a>sophagus is short, the rudimentary lungs appear by its sides, and the heart 

 lies just iii front. As the thorax is developed, however, the oesophagus becomes longer, 

 the lungs increase in size, and finally the diaphragm shuts off its cavity from the cavity 

 of the abdomen. The growth of the diaphragm is from its periphery to the central por- 

 tion, which latter gives passage to the vessels and the oesophagus. Sometimes, when this 

 closure is incomplete, we have the malformation known as congenital diaphragmatic 

 hernia. 



The development of the anus is sufficiently simple. At first, as we have seen, the 

 intestine terminates below in a blind extremity ; but, at about the seventh week, a lon- 

 gitudinal slit appears below the external organs of generation, by which the rectum 

 opens. This is the anus. It is not very unusual to observe an arrest in the development 

 of this opening, the intestine terminating in a blind extremity, a short distance beneath 

 the integument. This constitutes the malformation known as imperforate anus, a de- 

 formity which can usually be relieved, without much difficulty, by a surgical operation, if 

 the distance between the rectum and the skin be not too great. The opening of the anus 

 appears about a week after the opening of the mouth, at or about the seventh week. 



The rudiments of the liver appear very early, and, indeed, at the end of the first 

 month, this organ has attained an enormous size. Two projections, or buds, appear on 

 either side of the intestine, which form the two principal lobes of the liver. This organ 

 is at first symmetrical, the two lobes being of nearly the same size, with a median fis- 

 sure. One of these prolongations from the intestine becomes perforated and forms the 

 excretory duct, of which the gall-bladder, with its duct, is an appendage. During the 

 early part of fcatal life, the liver occupies the greatest part of the abdominal cavity. 

 According to Burdach, its weight, in proportion to the weight of the body at different 

 ages, is as follows : At the end of the first month, 1 to 3 ; at term, 1 to 18 ; in the 

 adult, 1 to 36. Its structure is very soft during the first months, and it is only at about 

 the fourth or fifth month that it assumes one of its most important functions, viz., the 

 production of sugar. As development advances, and as the relative size of the liver 

 gradually diminishes, its tissue becomes more solid. 



The pancreas appears at the left side of the duodenum, by the formation of two ducts 

 leading from the intestine, which branch and develop glandular structure at their ex- 

 tremities. The spleen is developed, about the same time, at the greater curvature of the 

 stomach. This organ is abundantly supplied with blood-vessels, but it has no excretory 

 duct. The spleen becomes distinct during the second month. 



There is no reason to believe that any of the digestive fluids are secreted during 

 intra-uterine life. The stomach, at least, never contains, at this time, an acid secretion. 

 At birth, the intestine contains a peculiar substance, called meconinm, which will be 

 described farther on. Cholesterine, an important constituent of the bile, is found in the 

 meconium in large quantity, but its function is connected exclusively with excretion. 



