30 PHYSIOLOGY. 



parts of the body, principally beneath the skin and the ab- 

 dominal muscles, and around the heart, kidneys, &c. while 

 none is ever found in the cranium, the brain, the eye, the 

 ear, the nose, and several other organs. 



12. The Serous Membranes.-fThese line all the closed 

 cavities, or sacs of the body, and are reflected over the or- 

 gans contained in them; Thus the cavities of the (brain and 

 chest, the abdomen and joints, are lined by serous membrane. 

 By its external surface, it is united to the walls of the cavity, 

 or the substance of the organ it invests ; by its internal sur- 

 face it is free and unattached ; whence this surface is in con- 

 tact only with itself, forming a close cavity, having no com- 

 munication with the external air. 



13. If it were possible therefore to dissect serous membranes 

 from off the parts which they invest, they would have the 

 form of a sac, without an opening, the organ invested by one 

 of these folds being altogether external to the cavity of that 

 sac ; just as happens when a double night-cap is worn, of 

 which the part immediately covering the head is analagoua 

 to that portion of the serous membrane which adheres to and 

 invests the organ, whilst the external portion of the cap 

 represents the lining of the cavity in which that organ is 

 said to be contained. Hence it will readily be understood 

 that the serous membranes never open, or allow of any per- 

 foration for the passage of blood-vessels, nerves, or ducts, to 

 or from the enclosed organs ; but that they are always re- 

 flected over them, forming a sheath around them, and accom- 

 panying them in their course. It also follows as a necessary 

 consequence, that their free surfaces completely isolate the 

 parts between which they intervene. 



144' The serous membranes are of a whitish, shining col- 

 our, and smooth on their free or inner surface* They are 

 kept constantly moist by a fluid which is exhaled in a 

 gaseous state from the serum of the blood, whence they 

 derive their name. They are also elastic and extensible, 

 and are said to be destitute of blood-vessels and nerves; 



