520 SYSTEM OP THE GREAT SYMPATHETIC. 



facial nerves, and are for the most part voluntary in their action. 

 The iris, on the other hand, is a more deeply-seated muscular 

 curtain, which regulates the quantity of light admitted through the 

 pupil. There is also the ciliary muscle, which regulates the position 

 of the crystalline lens, and secures a correct focusing of the light, 

 at different distances. Both these muscles are supplied, as we have 

 seen, by filaments from the ophthalmic ganglion, and their move- 

 ments are involuntary in character. 



In the olfactory apparatus, the anterior or superficial set of 

 muscles are the compressors and elevators of the alae nasi, which 

 are animated by filaments of the facial nerve. By their action, 

 odoriferous vapors, when faint and delicate in their character, are 

 snuffed up and directed into the upper part of the nasal passage?, 

 where they come in contact with the most sensitive portions of the 

 olfactory membrane; or, if too pungent or disagreeable in flavor, 

 are excluded from entrance. These muscles are not very im- 

 portant or active in the human subject; but in many of the lower 

 animals with a more active and powerful sense of smell, as, for 

 example, the carnivora, they may be seen to play a very important 

 part in the mechanism of olfaction. Furthermore, the levators and 

 depressors of the velum palati, which are more deeply situated, 

 serve to open or close the orifice of the posterior nares, and accom- 

 plish a similar office with the muscles already named in front. The 

 levator palati and azygos uvulas muscles, which, by their action, 

 tend to close the posterior nares, are supplied by filaments from the 

 spheno-palatine ganglion, and are involuntary in their character. 



The ear has two similar sets of muscles, similarly supplied. The 

 first, or superficial set, are those moving the external ear, viz., the 

 anterior, superior, and posterior auriculares. Like the muscles of 

 the anterior nares, they are comparatively inactive in man, but in 

 many of the lower animals are well developed and important. In 

 the horse, the deer, the sheep, &c., they turn the ear in various 

 directions so as to catch more distinctly faint and distant sounds, or 

 to exclude those which are harsh and disagreeable. These muscles 

 are supplied by filaments of the facial nerve, and are voluntary in 

 their action. 



The deep-seated set are the muscles of the middle ear. In order 

 to understand their action, we must recollect that sounds are trans- 

 mitted from the external to the middle ear through the membrane 

 of the tympanum, which vibrates, like the head of a drum, on 

 receiving sonorous impulses from without. 



