3lG SOLID PARTS OF ANIMALS. 



The mucous membrane, though apparently a continuation of 

 the skin, differs entirely from that tissue in its chemical proper- 

 ties. It is quite insoluble in water. When long boiled in that 

 liquid it becomes hard and brittle. Acids easily destroy it, and 

 convert it into a pap. It readily putrefies, and in that way its 

 texture is speedily destroyed. If we soften it in cold water, and 

 leave it in that state to the action of the atmosphere, it is con- 

 verted into a reddish mucous-looking substance before the other 

 coats of the intestines have begun to be affected. 



The mucous membrane, like the cutis, is covered by a very 

 thin epidermis, to which the term epithelium has been applied. 

 The chemical nature of this membrane has not been determined ; 

 but it is probably of the same nature with the epidermis. 



CHAPTER XXI. 



OF ARTERIES AND VEINS. 



L THE term artery* meant originally a tube containing air. 

 It was not till after the discovery of the circulation by Hervey 

 that their use was fully understood. They are tubes which con- 

 vey the blood from the heart to every part of the body, in order 

 to supply the waste of the system ; while the veins convey back 

 again to the heart all the blood which has not been consumed by 

 the different processes going on in every part of the body. 



An artery is a cylindrical and highly elastic tube, composed 

 of three coats placed one within the other. The external coat is 

 formed of the cellular tissue, into which it may be resolved by 

 maceration. Its texture is closer when it is in contact with the 

 middle coat, than externally when it is somewhat loose and floc- 

 culent It admits of considerable extension, and can retract 

 when the cause is removed, and it is so tough as not to be divided 

 by a hard ligature placed on the vessel, and so firm as alone to 

 resist the impulse of the current of blood, when the other coats 

 are divided or torn. 



The internal coat not only lines the arteries, but is continued 

 into the ventricles of the heart It is thin, homogeneous, trans- 

 parent, and so fragile as to be easily torn. It is considered by 



* From ttHf, air, and Tfiu>, / contain. 



