Dr. A. Waller^s Researches on the Organ of Vision. 519 



is too rapid to allow of the detection of the direction of the separate 

 particles of the blood. It is only when the circulation becomes lan- 

 guid that the separate globules can be seen distinctly running in a 

 centrifugal (i. e. arterial) direction. 



By compressing the eye shghtly, the passage of the blood may be 

 retarded, and by that means be easily followed ; but in so doing an 

 error may possibly be committed respecting the arterial nature of 

 these vessels, as the course of the blood is then generally reversed in 

 the arteries, and will be seen to take a centripetal direction, sometimes 

 for upwards of a minute, according to the amount of pressure. But 

 in a short time, after oscillating within the vessels, the blood again 

 resumes its natural course, which may be sufficiently regulated to 

 enable us to watch the passage of the globules in the oblique branches 

 and in their internal and external subdivisions. 



The veins of the iris form two layers. The superficial layer com- 

 prises all the larger veins — generally twenty-three or twenty-four 

 in number, — which radiate in a regular manner from the pupil out- 

 wards towards the ciUary ligament. They arise at the pupillary edge, 

 each by two or three fine twigs, which quickly meet in a common 

 trunk, or sometimes run separately as far as the outer half of the 

 iris, where they unite m a common trunk. 



The deep layer consists almost entirely of a fine network belong- 

 ing to the radiating muscular fibres, and presenting a close analogy 

 with the fine vessels supplying striated muscular fibre ; the vessels 

 being very minute, and the meshes elongated in the direction of the 

 fibres. Sometimes the vessels from this layer unite into a small 

 ramuscule, which empties into a radiating vein ; at others they unite 

 in a common trunk, passing beneath the ciliary ligament into the 

 choroid. 



The movement of the blood in the veins is generally not too rapid 

 to distinguish the direction of the current and the separate globules, 

 which appear to be constantly springing from around the edge of the 

 pupil and pouring outwards along the veins of the iris into the cho- 

 roidal and ciliary vessels. 



When the pupil is contracted, the radiating vessels are rectihnear ; 

 but when it dilates they become curved and bent into zigzag and 

 spiral forms, which are more or less curved or obtuse in proportion to 

 the degree of dilatation of the pupil. This change m the form of 

 the vessels does not appear to produce any difference in the speed of 

 the current of blood. 



Around the ciHary ligament are two and often three circular vessels, 

 receiving the blood from the conjunctiva of the cornea and sclerotic, 

 partly from the iris, and probably from the ciliary processes. Two 

 of them are venous, and empty themselves into four large veins, 

 corresponding to the anterior ciliaries, which arise in a perpendicular 

 direction, and after following a rectilinear course over the sclerotic, 

 finally end in the ophthalmic vem. The third circular ciliary vessel 

 is of an arterial nature, as shown by the greater thickness of its 

 parietes and the rapidity of its current. 



The current of blood in these vascular circles is a most interesting 



