DEVELOPMENT OF THE ALIMENTARY CANAL. 10T 



and the posterior or pelvic. From the first is formed the pharynx and oeso- 

 phagus ; from the second, the stomach, small intestine and large intestine, as far 

 as the upper part of the rectum ; from the third, the middle third of the rec- 

 tum. The buccal cavity, on the one hand, and the lower portion of the rectum 

 on the other, are separate productions from the external layer of the blasto- 

 dermic membrane, and do not communicate with the common cavity till a later 

 period. The permanence of the foetal septum in either case constitutes a well 

 _ known deformity imperforate oesophagus or imperforate rectum, as the case 

 may be. The anal cavity is at first common to the urogenital, as well as to the 

 digestive organs. 



The development of the palate has been spoken of above. 



The tongue appears about the fifth week as a small elevation, behind the 

 inferior maxillary arch, to which is united another projection from the second 

 pharyngeal arch. The epithelial layer is furnished by the external blastoder- 

 mic membrane. 



The tonsils appear about the fourth month. 



The middle portion of the primitive intestine is at first a straight tube, com- 

 municating freely with the umbilical vesicle. It then leaves the vertebral 

 column in the middle, and forms a curve attached to that column by the mesen- 

 tery. A portion of the intestine above this mesentery dilates into the stomach, 

 which gradually also acquires a mesentery of its own; the rest remains at- 

 tached to the spine, and forms the duodenum. The curve of the intestine ap- 

 pears as it were drawn out from the body by its attachment to the vitelline 

 duct, and lies external to the parietes, and in the umbilical cord, until the end 

 of the third month, when it passes back again into the abdomen. While still 

 forming a portion of the cord, the intestine begins to be distinguished into large 

 and small, for the anterior or upper part, corresponding to the small intestine, 

 begins to assume a convoluted arrangement about the eighth week, whilst the 

 lower part, which had been posterior, passes to the front and right side of the 

 other, and becomes dilated at a short -distance from the insertion of the vitelline 

 duct, to form the rudiment of the caecum. When the intestine lies wholly in 

 the belly, the curve of the large intestine begins rapidly 'to form; but the 

 cascum lies for some time in the middle line, and the ascending colon is not 

 fully formed till the sixth month. 



The source of each layer of the intestine, and the closure of the omphalo- 

 mesenteric or vitelline duct have been spoken of (above pp. 91, 94). 



The liver appears after the Wolffian bodies, about the third week, in the form 

 of two depressions formed by the epithelial and fibro-intestinal layers of the 

 blastodermic membrane, and projecting from the intestine at the part which 

 afterwards forms the duodenum. These depressions are developed into the 

 right and left lobes. They grow very rapidly around the omphalo-raeseuteric 

 vein, from which they receive the branches enumerated on p. 106, and about 

 the third month the liver almost fills the abdominal cavity. From this period 

 the relative development of the liver is less active, more especially that of the 

 left lobe, which now becomes smaller than the right ; but the liver remains up 

 to the end of foetal life relatively larger than in the adult. 



The gall-bladder appears about the second month, and bile is detected in the 

 intestine in the third month. 



The pancreas is also an early formation, being far advanced in the second 

 month. It, as well as the other salivary glands, which appear about the same 

 period, originates in a projection from the epithelial layer, which afterwards 

 forms a cavhy, from the ramifications of which the lobules of the gland are 

 formed. 



Development of the Respiratory Organs, The lungs appear somewhat later 

 than the liver. They are developed from a small cul-de-sac which is formed 

 on either side as a projection from the epithelial and fibrous laminae of the in- 



