458 



MANUAL OF HISTOLOGY. 



ends of the branches (2, a) there then appear round vesicular dilatations 

 (6), lined with cylindrical cells (3, a), which by a process of gemmation 

 are resolved into a number of finer loculi, from which then the infundi- 

 bula or primary lobules are derived, and in all probability also their air- 

 vesicles by a further sacculation of their walls. 



The pulmonary tissue is subject to many changes. One senile meta- 

 morphosis consists in the disappearance of parts of the alveolar walls, and 

 confluence of the air vesicles to form larger cavities, with consequent 

 destruction of the capillaries contained in the interalveolar septa. 



The occurrence, of new growths here is an obscure subject, especially 

 as regards their point of origin. This may probably be the nuclei of 

 the vascular cells, or the epithelium of the lung. 



3. The Digestive Apparatus. 

 244. 



The digestive apparatus consists of the mouth with its teeth, already 

 described^ 150 and 156, the tongue, and attached salivary glands, then 

 of the pharynx, oesophagus, stomach, large and small intestine, the large 



glands emptying their secretions 

 into the upper portion of the last 

 of these, namely, the pancreas and 

 liver. Almost every variety of 

 tissue takes part in the formation 

 of this extensive group of organs, in 

 which the glandular elements espe- 

 cially are pre-eminently important, 

 and are found from the upper to 

 the lower aperture of the alimentary 

 canal forming a mucous covering 

 for the whole internal surface. 



The cavity of the mouth is lined 

 by a mucous membrane of the texture 

 roughly described already ( 136), 

 which is marked on its free surface 

 by a multitude of closely crowded 

 conical and filiform papillae (fig. 

 440). The thickness of this mem- 

 brane varies, its maximum being 

 sometimes 0'45 mm. The papillae likewise differ greatly in length, rang- 

 ing from 0*23 mm. to - 45 mm. The strongly laminated epithelial layer 

 consists of flattened cells (fig. 444), which we have already considered at 

 greater length at p. 141. At the opening of the mouth they are con- 

 tinuous with the cells of the epidermis. 



This mucous membrane itself is rich in elastic fibres, and presents a 

 network of connective-tissue bundles. It is denser towards the surface, 

 upon which a homogeneous transparent limiting layer may be seen. In 

 the papillae here, as in those of the external skin, and still more in the 

 villi of the intestine, the connective-tissue loses its fibrous character more 

 or less, and presents itself in a rather undeveloped form. 



Below, the mucous membrane gradually merges into submucous tissue. 

 The latter is in some localities a solid fibrous mass, as in the case of the 

 gum, and in others soft and elastic, with loose texture, as on the floor of 



Fig. 440. A papilla from the gum of an infant, 

 with its vascular network and epithelial cover- 

 ing. 



