LESSON 29.] THE OKGANS OF NUTRITION. 97 



486. Serous membranes invest the abdominal viscera, and pass 

 from one viscus to another, until they have invested the whole of 

 them, when they are reflected on the parieties (sides). 



487. The Synovial membrane is a thin membranous layer, which 

 covers the articular (joints) surface of the bones, from which it is 

 reflected upon the surfaces of the ligaments which surround and enter 

 into the composition of a joint. Like the serous membranes, these also 

 are shut sacs ; the peculiar fluid secreted by them is called synovia. 



488. Certain sacs surrounding some of the joints are called 

 bursce mucosce (mucous purses) ; these are shut sacs, allied in struc- 

 ture to synovial membranes, and secreting a synovial fluid. 



LESSON XXIX. 



THE ORGANS OF NUTRITION. 



489. If, during the summer months, a drop of water containing 

 animal or vegetable matter, or both, in a state of decomposition, be 

 examined with a microscope, it will display a world of animated 

 atoms ! 



These compose the individuals forming the class known as " mi- 

 croscopical animal cula" The form and size, no less than other 

 characteristics of the individuals of this class, is extremely various ; 

 some of them (Monas crepusculus) being so minute that a single 

 drop of water would contain five hundred millions of them. Our 

 present object, however, is to inquire into the particulars of their 

 nutrition. 



490. Prof. Ehrenberg long ago promulgated the idea that the 



majority of these creatures are endowed with a 



FIG. 165. 

 great but variable number of digestive sacs, each 



of which (according to him) is independent of the 

 rest, but all of them communicating directly with 

 the oral cavity (Fig. 165, 6), as shown in the pro- 

 fessor's figure of the Monas termo supposed to 

 be the most minute animal revealed to our vision 

 even by the microscope. 



491. Increased knowledge of these creatures 

 may probably demonstrate some mistake in con- 

 nection with this description, which is at variance with all that we 

 really know ; the figures (of the same authority) of larger animals 



7 



