248 SENSE OP SIGHT. 



and by M. Magendie, 1 that nictation is effected under the influence 

 chiefly of the portio dura of the seventh pair or facial nerve, one of 

 the respiratory nerves of Sir Charles Bell's system the respiratory 

 of the face. When this nerve is cut, nictation is completely arrested ; 

 and when the nerve of the fifth pair, also distributed to these parts, is 

 divided, it ceases likewise, but less thoroughly; a very vivid light 

 exciting it, but only at considerable intervals, and imperfectly. We 

 see here something very analogous to the partition of the nerves of the 

 senses into those possessing general, and special sensibility. Like the 

 latter functionaries, the nerve of the seventh pair appears to be specially 

 concerned in nictation, and not to be capable of executing its office, 

 unless the fifth pair the nerve of general sensibility be in a state of 

 integrity. The explanation of Dr. Marshall Hall is different. It has 

 been before remarked, that if the functions of the brain be suspended 

 or destroyed, the true spinal system being uninjured, the orbicularis 

 palpebrarum still contracts so as to close the eyelids, when the tarsus 

 is touched with any solid body. In this case, neither sensation nor 

 volition can be concerned. It is a reflex action; the excito* nerves 

 being probably branches of the fifth, and the motor, branches of the 

 seventh pair. Hence, when the will ceases to act, as in sleep, or in 

 apoplexy, the lids close over the eye to protect it. In the waking 

 state, the levator palpebrae, under the influence of the will, acts as an 

 antagonist to the orbicularis and keeps the eye open; but there is an 

 almost irresistible tendency to close the eye; and, as in the case of 

 respiration, the muscular contraction can only be restrained to a cer- 

 tain degree : it takes place, whenever the condition of the conjunctiva 

 is such as to occasion an impression to be conveyed along the excitor 

 nerve which demands a reflex movement to modify it ; for example, 

 when particles of dust collect upon it ; or the surface becomes dry. 2 



The eyelids, by their approximation, can regulate the quantity of 

 light that enters the pupil, when it is injuriously powerful ; when feeble, 

 they are widely separated, to allow as much light as possible to pene- 

 trate the organ. By their agency, again, the most diverging rays from 

 an object can be prevented from falling upon the cornea; and the vision 

 of the myopic or short-sighted can be assisted. It is a means of which 

 they often avail themselves. The cilia or eyelashes, it is probable, are 

 of similar advantage as regards the admission of light into the eye, and, 

 probably, have some part in preventing extraneous bodies, borne about 

 in the air, from reaching the sensible conjunctiva. 



The muscles of the eyeball have acquired the chief portion of their 

 interest in recent times, and largely through the investigations of the 

 eminent physiologist of whose labours we have so frequently had 

 occasion to speak Sir Charles Bell. 3 The arrangement of the four 

 straight muscles, and especially their names, sufficiently indicate the 

 direction in which they are capable of moving the organ, when acting 

 singly. If any two of them contract together, the eyeball will, of 



1 Precis Elementaire, i. 309. 



a Carpenter, Human Physiology, p. 154, London, 1842. 



3 Op. chat., p. 102, and Anatomy and Physiology, 5th Amer. edit, ii. 213, New York, 

 1827. 



