SOLID PARTS OF ANIMALS. 



equalities in the surface of these organs may be distinctly seen 

 through the serous membranes that invest them. 



Serous membranes are capable of very considerable distension, 

 as is obvious in dropsy and in the various hernia? of the intestines. 

 In their natural state they are insensible or nearly so ; but when 

 they are inflamed, acute pain is felt in them. As they pass from 

 one viscus to another, it is obvious that they must form folds ; 

 and these folds have been often distinguished by names, as amen- 

 tum, mesentery., mesocolon, mediastinum, &c. 



Blood-vessels may be seen entering into the serous membranes 

 in cases of inflammation. Hence it follows that they are supplied 

 with arteries and veins. Whether they possess exhalent vessels 

 to throw out the serum or lymph with which they are moistened, 

 has not been ascertained. 



As to the chemical nature and properties of the serous mem- 

 branes, no experiments, so far as I know, have hitherto been 

 made upon the subject. It is stated in chemical books that when 

 boiled in water they are converted into collin. Hence it has 

 been inferred that they are merely inspissated cellular membrane. 

 But I am not aware of any person having tried the experiment. 

 It is certain that the small intestines may be boiled for a long 

 time without being deprived of their outer serous coat, and with- 

 out that coat undergoing any sensible change. 



The liquid exhaled from the surface of the serous membranes 

 will be described, and its constituents stated in a succeeding 

 chapter of this work when treating of lymph. 



CHAPTER XX. 



OF MUCOUS MEMBRANES. 



THE mucous membrane, in an anatomical point of view, may 

 be considered as one continuous membrane prolonged from the 

 integuments into the interior of the passages of the body, where 

 it serves a corresponding purpose with the skin ; but which, from 

 the nature of the fluid which it secretes, and which covers it, has 

 received the name of mucous membrane. From the lips and nos- 

 trils it extends along the whole length of the alimentary canal as 

 well as into the different follicles and excretory ducts which open 



