SYSTEMATIC SYNTHESIS OF TISSUES INTO OEGANS. 311 



one arm to the external membrane, and by its head is 

 jointed to the anvil, which by one arm is jointed in 

 early life, but is soon consolidated with the round bone 

 that is jointed to the stirrup attached to the oval window. 



857. Three ligaments SUSPEND THESE BONES in 

 their places, and three delicate muscles CONTROL THEIR 

 MOVEMENTS, tightening or relaxing the chain of bones, 

 and of course the opposite membranes to which they 

 are attached. 



858. THE DRUM is LINED by a very delicate mem- 

 brane, that is continued over its chain of bones as well : 

 the inner surface of the membrane is constructed of cells. 



859. THE MASTOID CELLS are numerous small cavi- 

 ties in the Mastoid process, just back of the ear, opening 

 into the back part of the drum, and lined with a similar 

 membrane. Their use is not known. 



860. THE EUSTACHIAN TUBE is one in which the 

 lower part of the drum terminates : it narrows till not 

 larger than a crow-quill ; then enlarging, opens into the 

 side of the upper and back part of the nostril, and of the 

 pharynx. 



861. THE USE OF THE EUSTACHIAN TUBE is to allow 

 air to pass into the drum; and when it is closed by a cold 

 or disease, hearing is lost. It also serves as a drain to 

 the drum and mastoid cells, and is sometimes closed by 

 collections from them. 



862. THE INTERNAL EAR (labyrinth) is CONSTRUCTED 

 of a vestibule, semicircular canals, and cochlea. 



863. THE VESTIBULE is an irregular, small, oval cav- 

 ity, wrought into the bone; into it the oval window 

 would open if the stirrup should be removed. It is 

 directly within the drum. It is lined with a delicate 

 membrane, adhering to the bone on one side, and finish- 

 ed with cells on the other, and is filled with serous fluid 

 called peri-lymph. 



857. What ? 85S. How is ? 859. What are ? 860. What is ? 861. What 

 is ? 862. How is ? 863. What is ? 



