426 ORGANISATION OF THE IRIS. 



tion has been unattended with the effusion of a single drop of 

 blood, and the restoration to sight was immediate. 

 The iris is extremely vascular, and receives its blood partly 

 from the long, and partly from the anterior ciliary arteries. 

 They anastomose with each other, so as to form the greater 

 arterial circle round the outer margin of the iris, and then run 

 out in a serpentine course ^and from a lesser circle near the 

 pupillary margin. From this lesser circle, innumerable minute 

 vessels radiate towards the centre, and abruptly terminate at 

 the edge of the pupil. When the pupil is contracted to its 

 minimum size, these vessels become straight. The veins are 

 also abundant ; some terminate in the long ciliary veins, and 

 some in the vasa vorticosa. 



With nerves also, the iris is richly furnished, probably more 

 abundantly than any other tissue of equal size in the whole 

 economy. It is supplied from the ciliary nerves, which run 

 out between the choroid and sclerotica in the form of flattened 

 cords, after they have perforated obliquely the latter tunic. 

 They inosculate with each other frequently in the greater 

 arterial circle of the iris, and appear under the microscope to 

 form little knots or ganglia ; the branches which they send 

 inwards towards the pupil, may be seen in some of the larger 

 quadrupeds to inosculate with each other, but cannot be traced 

 satisfactorily on account of their faint appearance and pulpy 

 character. 



In the foetus, prior to the seventh month after conception, 

 the pupil is closed by a delicate vascular membrane, called 

 the Membrana Pupillaris ; which, after this period, com- 

 pletely disappears.* 



The membrana pupillaris as shown by W. Hunter and 

 J. Cloquet, consists of two layers, formed by an extension, 

 back to back, of the serous lining membrane of the anterior 

 and posterior aqueous chambers, across the pupil. This is 

 ruptured, or effaced by a retraction of its vessels, so as to 



* The discovery of the Membrana Pupillaris is claimed by Wachendorf, 

 Prof, of Botany in the University of Utrecht ; by John Hunter ; by Haller j 

 and by Albinus. 



