920 EMBRYOLOGY. 



The middle intestine appears at first as a uniform cylindrical tube, whose 

 diameter is afterwards modified to constitute the organs comprised between 

 the oesophagus and rectum. 



1. Mouth. It begins by a depression limited by the maxillary buds; 

 this blind pouch gradually enlarges inwardly, and proceeds to meet the 

 pharynx, from which it is only separated, at a certain period, by a thin 

 membrane ; this is at last absorbed, and the two cavities then communicate. 

 Until the third month, the mouth is confounded with the nasal cavities ; at 

 this time the palate appears, and eventually isolates them. 



2. Tongue. Appearing at first as a small prominence from the maxillary 

 buds, the tongue is completed by the addition to it of a bud from the second 

 branchial arch. Its epithelium and glands come from the external blas- 

 todermic lamina ; they are developed in the third and fourth months. 



3. Pharynx and (Esophagus. These two organs become enlarged and 

 elongated as the foetus grows. The oesophagus communicates at first with the 

 trachea ; but it slowly closes, and finally separates completely from that canal. 



4. Stomach. This is formed by the dilatation of the anterior part of the 

 middle intestine ; this dilatation is fusiform, and its largest axis lon- 

 gitudinal ; it soon incur vates, and its longest axis is then transversal. In 

 Kuminants, the stomach is at its first appearance simple ; but before long it 

 shows grooves on its surface, and in its interior septa, as in the normal 

 state. During foetal life this organ is small ; but after birth, when solid 

 food begins to be taken, it augments rapidly in volume. During lactation 

 in Kuminants, there is remarked a predominance of the fourth over the 

 other gastric compartments ; but immediately the young animal commences 

 to consume fibrous aliment, the rumen quickly increases in size, and it is 

 not long before it becomes the most considerable division. 



5. Intestines. The intestinal tube is primarily of a uniform calibre, 

 though in a short time there can be distinguished the various regions of 

 which it is composed. According to A. Baer, the cascum is early seen in 

 hoofed animals, and is situated in- the vicinity of the omphalo-mesenteric 

 duct. This duct detaches itself from the extremity of an intestinal loop, 

 which is drawn towards the umbilical ring ; when the latter is becoming 

 atrophied and progressing towards complete obliteration, this loop re-ascends 

 into the abdominal cavity. 



The intestines are smooth on their inner face during the first two 

 months ; but during the third they show their villosities and the glands of 

 Lieberkiihn. The Brunnerian and solitary glands are a little later in 

 showing themselves. 



6. Rectum. This is derived from the posterior intestine, and is de- 

 veloped like the other portions. 



7. Anus. Towards the caudal extremity of the foetus is observed a 

 depression analogous to the buccal cul-de-sac. This gradually deepens, and 

 is joined to the rectum and genitourinary organs. Later, it is separated 

 from the latter, and then belongs exclusively to the alimentary canal. 



B. ANNEXES OF THE ALIMENTARY CANAL. These are the salivary glands, 

 teeth, liver, pancreas, and spleen. 



1. Salivary glands. These are developed in a solid cellular bud, which 

 is related to the epithelium at the commencement of the digestive apparatus. 

 This bud increases, and at the same time is hollowed into glandular pouches. 

 The submaxillary gland is the first to appear ; according to Bischoff, it is 

 entirely formed in the foetus of a Cow only an inch in length. The parotid 

 gland is the last to be formed. 



