chap, iv.] THE ORBIT AND EYE. 53 



more or less from its attachments without any damage 

 being done to the other tunics. The iris has been 

 completely torn away in a few instances, and has 

 escaped through a wound of the globe. The ciliary 

 processes have been thus exposed. Congenital absence 

 of the iris has been recorded. In cases of penetrating 

 wounds of the cornea the iris is easily prolapsed. 

 It is so delicate and yielding a membrane, that in per- 

 forming iridectomy the necessary piece of the iris can 

 be seized and pulled out through the corneal incision 

 without offering sensible resistance. The membrane 

 also derives much support from its contact with the 

 lens, for in cases where the lens has been displaced 

 into the vitreous or has been removed by operation, 

 the iris is observed to be tremulous when the globe is 

 moved. Although very vascular, the iris seldom 

 bleeds much when cut, a circumstance that is pro- 

 bably due to the contraction of the muscular fibres 

 that exist so plentifully within it. Sometimes the 

 iris presents a congenital gap in its substance that 

 runs from the pupil downwards and a little inwards. 

 This condition is known as coloboma iridis, and is due 

 to the persistence of the "choroidal cleft." In other 

 cases there can be seen, stretching across the pupil, 

 some shreds of the pupillary membrane. Normally 

 this membrane, which is apparent for a few days after 

 birth in some animals, is entirely absorbed before birth. 



It will now be convenient to take note of the 

 blood and nerve supply of the globe. 



Blood supply of the eye-ball. 1. The short 

 ciliary arteries (from the ophthalmic) pierce the sclerotic 

 close to the optic nerve, run some little way in the 

 outer coat of the choroid, and then break up into a 

 capillary plexus that makes up the main part of the 

 inner choroidal coat. In front this plexus gives some 

 vessels to the ciliary processes. The veins from these 

 vessels are disposed in curves as they converge to four 



