CHAP. L] TISSUES AND MECHANISMS OF DIGESTION. 417 



also resembles the skin, but in the latter structure the papillae are 

 more abundant and more regular in form and size. 



The dermis, or connective tissue basis of the epithelium, is a 

 network of fibres and fine bundles of connective tissue, with 

 connective tissue corpuscles and a considerable number of fine 

 elastic fibres ; the number of leucocytes in the meshes of the 

 network is relatively scanty. A few scattered masses of retiform 

 or adenoid tissue, of which we shall speak later on, occur here -and 

 there. 



The mucous membrane proper is defined from the underlying 

 submucous tissue by a muscularis mucosse of plain unstriated 

 muscular fibres, lying at some distance from the epithelium. 

 These muscular fibres are absent at the upper part of the 

 oesophagus, appear lower down in isolated longitudinal bundles, 

 and eventually form a distinct layer, which however is not so 

 regular as in the rest of the alimentary canal, and consists of 

 longitudinal fibres only, circular fibres being absent. 



In man a few but in other animals a considerable number of 

 small 'mucous' and 'albuminous' glands are found in the submucous 

 tissue ; their ducts, penetrating the muscularis mucosse where 

 present, open on to the surface of the mucous membrane. In man 

 and mammalia these glands appear to serve only the purpose of 

 keeping the internal surface of the oesophagus moist ; but in some 

 animals, as in the frog, in which the epithelium of the oesophagus 

 is not the many layered stratified epithelium just described, but 

 a single layer of columnar ciliated cells mixed with mucous cells, 

 of the kind which we shall later on describe as ' goblet ' cells, there 

 is a large development of glands at the lower part of the oeso- 

 phagus, and the cells of these glands manufacture pepsin. 



As in other parts of the alimentary canal the submucous tissue 

 carries the larger blood vessels whose smaller branches supply the 

 mucous membrane; and lymphatics, beginning in the mucous 

 membrane, form considerable plexuses in the submucous coat. 



222. In man both the thicker inner circular and the outer 

 thinner longitudinal muscular layer consist in the upper part of 

 the oesophagus exclusively of bundles of striated fibres, which in 

 their main characters are identical with ordinary fibres of skeletal 

 muscles. At about the end of the upper third or sooner, bundles 

 of plain unstriated fibres make their appearance among the 

 bundles of striated fibres, and a little lower down the striated 

 fibres disappear, so that, in the lower half or more of the tube, 

 both circular and longitudinal layers are composed almost exclu- 

 sively of plain unstriated fibres, a few stray bundles of striated 

 muscle being found here and there. The relation of the striated 

 and unstriated fibres differs however in different animals ; in some 

 the striated tissue reaches down nearly to the stomach. 



Above, both longitudinal and circular layers merge into the 

 inferior constrictor of the pharynx; below, the longitudinal 



