SENSITIVE SYSTEM. 385 



The ciliary nerves lavish their branches upon the iris ; but, owing 

 to their minuteness, defy all attempts to make out their distri- 

 bution. 



Motion. — The motions of the iris consist in contraction and 

 dilatation, and the dimensions of the pupil are conversely regu- 

 lated thereby : when the iris becomes contracted the pupil be- 

 comes dilated, and vice versa; the pupil, however, is never shut 

 so completely as to exclude light altogether. These motions are 

 involuntary — excited by the presence of light — and, ceteris pat i- 

 bus, are regulated by the quantity admitted, or the suddenness 

 with which it is admitted, into the eye. This action, however, 

 is not the effect of direct stimulus, but proves to be a secondary 

 or sympathetic result, since light has no visible influence upon 

 the iris itself — the impression is made upon the retina: in gutta 

 serena (paralysis of the optic nerve), for example, the pupil be- 

 comes dilated and the iris remains motionless ; and when the 

 pupil is closed so that light cannot get to the bottom of the eye, 

 the iris becomes fixed. Furthermore, the motions of the iris are 

 influenced by the magnitude and distance of the object of vision : 

 when the eye is intently viewing a near object, or one very small, 

 the pupil becomes contracted j but if the object be removed to 

 a distance, or it have a wide surface, the pupil will become di- 

 lated. During sleep the pupil is contracted. After death it re- 

 mains in that state (either contracted or dilated) in which it hap- 

 pened to be at the instant of the last expiration, and cannot by 

 any means we may employ be re-excited to action. The efficient 

 cause of these movements remains undeveloped. 



Corpora Nigra. 



I have given this name to the little globular, black bodies 

 found attached around the pupillary margin of the iris ; and I 

 have taken the liberty to do so, because I am not aware that any 

 appellation has been assigned them by the veterinarians of this 

 country. Girard, speaking of them, says, " que I'ouverture pu- 

 pillaire ofFre commumtneiit quelques tubercles noirs, sortes de 

 prolongemens franges, replies en dehors, et nommes fungus ;^^ 

 but the termjungus sounds to our ears so much like disease, that 

 I consider this a sufficient reason for not adopting it. By this 

 laconic description of Girard's (for this is all he says about them) 

 it would appear that they are not uuexceptionably present: I 

 have not myself remarked their deficiency unless where the pig- 

 ment was wanting. There are commonly three of them, about 

 the magnitude of peppercorns, ranged en masse along the upper 

 margin of the pupil, something nearer to the outer than the 

 inner corner; and the largest is placed outvvardmost — unless 

 3 D 



