j.j 



; 



in 



: 



THE STRUCTURE OF THE EYEBALL 509 



(4) Contraction of the pupils is a marked effect of certain drugs such as 

 Morphia or its crude extract Opium ; other examples are Pilocarpine, Mus- 

 carine, Physostigmine and Cocaine. The parts of the pupillo- motor mechan- 

 ism on which these drugs act will be considered later (see page 513). 



(5) Constricted pupils are also met with in excitable conditions of the 

 central nervous system, and therefore during the induction of 

 chloroform and other anaesthesia. 



(6) Small pupils which do not react to light are also met with in injuries 

 to the spinal cord which involve the cervical region. The explanation of 

 this will be given later (see page 511). 



(7) Contracted pupils are found to accompany agony. This is probably 

 due to the powerful flow of efferent impulses which leave the brain in this 

 condition, affecting the 3rd nerve nucleus which controls the pupil. 



(8) The pupil contracts when the aqueous is allowed to escape 

 from the anterior chamber. The cause of this is said to be the dilatation 

 of the vessels of the iris, owing to the fall of the surrounding pressure. 



DILATATION OF THE PUPIL (1) Occurs on removal of a light stimulus 

 from the eyes. If the removal be complete the pupil remains dilated, but 

 if there be any light at all the pupil gradually contracts again as the eye 

 becomes dark adapted. 



(2) Occurs on accommodation for distant vision because the associated 

 reflex stimulation of the pupilo-motor centre with accommodation is no 

 longer called into play. 



(3) Reflex dilatation of the pupil can be excited by the stimulation of 

 any sensory-nerve. This may be due to some of the afferent impulses reach- 

 ing the cilio-spinal sympathetic nerve centre in the cord. 



(4) The pupils are frequently found to dilate in such emotional states as 

 fear, anxiety, exhaustion and dyspnoea, and also at the moment of death. 



(5) Dilatation is also found to accompany extreme exhaustion of the 

 central nervous system, when the activity of all nerve centres is low, 

 such as in deep chloroform anaesthesia, and in the coma produced by 

 alcohol poisoning. Many other drugs such as atropine and homatropine 

 cause dilatation, as will be described later. 



(6) Dilated pupils inactive to light are found in injuries of the 3rd nerve, 

 or its nucleus. 



(7) Dilated pupils are also found when the intraocular pressure is abnor- 

 mally high, as in glaucoma. This appears to be due to constriction of the 

 vessels of the iris owing to the high external pressure to which they are 



ubjected. 



(8) Dilated pupils inactive to light are found in compression and severe 

 concussion of the brain. This is probably due to the abolition of the normal 

 nervous impulses to the muscles, so that the pupil dilates under the 

 influence of its radial elastic fibres. 



1(9) Dilated pupils are found to accompany hyperactivity of the supra- 

 nal glands, owing to the presence of considerable amounts of adrenaline 

 the blood. This occurs for example in oxygen want, dilated pupils being 

 one of the characteristic signs of that condition. 



