588 THE SYMPATHETIC SYSTEM. 



are mainly voluntary in their action. The iris, on the other hand, is a 

 deep-seated muscular curtain, which regulates the quantity of light 

 admitted through the pupil. It is supplied by filaments from the oph- 

 thalmic ganglion, and its movements are involuntary. 



Division of the sympathetic nerve in the middle of the neck has a 

 marked effect on the muscular apparatus of the eye. Within a few 

 seconds after this operation has been performed upon the cat, the pupil 

 of the corresponding eye becomes contracted, and remains in that con- 

 dition. At the same time the third 

 Fig. 185. .. . t . t . 



eyelid, or " nictitating membrane," 



with which these animals are pro- 

 vided, is drawn partially over the 

 cornea, and the upper and lower 

 eyelids also approximate to each 

 other; so that all the apertures 

 guarding the eyeball are percep- 

 tibly narrowed, and the expression 

 of the face on that side is altered 

 in a corresponding degree. This 

 effect has been explained by sup- 

 posing the circular fibres of the 



OAT, after section of the right sympathetic. . . , . , , 



iris, or the constrictors, to be ani- 

 mated by filaments derived from the oculomotorius, and the radiating 

 fibres, or the dilators, to be supplied by the sympathetic; so that, while 

 division of the oculomotorius would produce dilatation of the pupil by 

 paralysis of the circular fibres only, division of the sympathetic would 

 be followed by exclusive paralysis of the dilators, and consequently by 

 contraction of the pupil. This explanation, however, is not entirely 

 satisfactory ; since, after division of the sympathetic nerve in the cat, 

 not only is the pupil contracted, but both the upper and lower eyelids 

 and the nictitating membrane are also drawn partially over the cornea, 

 and assist in excluding the light. The last-named effect cannot be 

 owing to direct paralysis, from division of the fibres of the sjonpathetic. 

 It is more probable that the section of this nerve operates by exaggerat- 

 ing for a time the sensibility of the retina, owing to vascular congestion; 

 and that the partial closure of the eyelids and pupil is a consequence 

 of that condition. 



In the olfactory apparatus, the superficial set of muscles are the com- 

 pressors and elevators of the alee nasi, which are animated by filaments 

 of the facial nerve. By their action, odoriferous vapors are snuffed up 

 and directed into the upper part of the nasal passages, where they come 

 in contact with the sensitive portions of the olfactory membrane ; or, 

 if too pungent or disagreeable in flavor, are excluded from entrance. 

 These muscles are not very important in the human species ; but in 

 many of the lower animals, as in the carnivora, they play a very im- 

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

 and depressors of the velum palati, which are deep-seated, serve to open 



