THE ORAL CAVITY. 235 



TECHNIC (CIRCULATORY SYSTEM). 



To obtain a topographical view of the layers composing the 

 heart and vessels, sections are made of tissues that have been fixed in 

 Miiller's fluid, chromic acid, etc. If the specimens are'to be studied in 

 detail, small pieces must be used, and are best fixed in chromic-osmic 

 mixtures or corrosive sublimate. Celloidin imbedding is recommended 

 for general topographic work. The further treatment is elective. 



The endothelium of the intima may be brought to view by silver 

 nitrate impregnation methods, by injecting silver solutions into the vascu- 

 lar system. The endothelial elements of the smallest vessels and capil- 

 laries are then clearly defined by lines of silver. Larger vessels must 

 be cut open, the intima separated, and pieces of its lamellae examined. 



Elastic elements, plates and networks are best observed in the 

 tunica media of the vessels, very small pieces of which are treated for 

 some hours with 33% potassium hydrate. 



The appropriate stains for sectionwork are those which bring 

 out the elastic elements and the smooth muscle-cells. For the former, 

 orcein is used. 



For demonstrating the distribution of the capillaries, the reader is 

 referred to the injection methods. The lymph -capillaries are injected 

 by puncture ; compare also the methods of Altmann. 



III. THE DIGESTIVE ORGANS. 



THE intestinal canal with the glands derived therefrom originates 

 from the inner layer of the blastoderm, the entoderm. The latter, 

 however, does not extend to the external openings of the body, as 

 the ectoderm forms depressions at these points which grow inward 

 toward the still imperforate fore and hind gut to communicate 

 finally with its lumen. This applies as well to the formation of 

 the primitive oral cavity, which is separated only secondarily into 

 oral and nasal cavities, as to the anus. The anterior boundary 

 between the ectodermal and entodermal portions of the digestive 

 tube consists of a plane passing through the opening of the pos- 

 terior nares and continued downward along the palatopharyngeal 

 arch. Everything lying anterior to this is of ectodermal origin, 

 therefore the entire oral and nasal cavities with their derivatives. 

 The lining of these cavities consists, however, of a true mucous 

 membrane, closely resembling in its structure that of the intestinal 

 tract. 



A. THE ORAL CAVITY. 



The epithelium of the oral mucous membrane is of the stratified 

 squamous type, differing from the epithelium of the epidermis 

 in that the stratum granulosum does not appear here as an inde- 

 pendent layer. The stratum lucidum is also wanting, and the 



