VESSELS OF THE IRIS. 



mammals. It is admitted, however, to be well developed in birds (where like the intraocular 

 muscular tissue generally it is of the striped variety) and, amongst mammals, in the otter. 

 Other writers have strenuously contended for the existence of a dilatator muscle. I have 

 myself. obtained unmistakeable evidence of the presence of a thin layer of fibres at the back of 

 the iris, under the pigment-cells, having all the appearance of flat plain muscle-cells. Recently 

 physiological proof of the existence of dilatator fibres has been furnished in the cat and 

 other animals by the experiments of Langley and Anderson. 



The choroidal part of the iris is often described as bein^ bounded on its anterior and 



Fig. 40. SEGMENT OF THE IRIS, SEEN FROM THE 



POSTERIOR SURFACE AFTER REMOVAL OF TIIK 

 t VEAL PIGMENT (Iwanoff). 



a, sphincter muscle ; 6, dilatator muscle of the 

 pupil. 



posterior surfaces by special layers, which have 



been termed the anterior and posterior limiting 



or hyaline layers. Of these, the anterior layer 



is merely the superficial part of the general 



stroma, having the fibres and cells somewhat 



more closely arranged ; and what has usually 



been described as the posterior is probably the 



thin muscular sheet of the dilatator pupillce. 



There may, however, be a thin membrane corresponding to the membrane of Bruch of the 



choroid immediately underlying the epithelium of the pars retinalis. 



Vessels of the iris (figs. 30 to 34, and fig. 41). The long ciliary arteries, 

 two in number, pierce the sclerotic a little in advance, and one on each side, of 

 the optic nerve. Having gained the interval between the sclerotic and choroid 

 coats, they extend horizontally forwards covered by loose connective tissue to the 

 ciliary muscle. In this course they lie nearly in the horizontal plane of the axis of 

 the eye-ball, the outer vessel being a little above, and the inner one a little below 

 the level of that line. A little behind the attached margin of the iris, each vessel 



Fig. 41. FRONT VIEW OF THE BLOOD-VESSELS 



OF THE CHOROID COAT AND IRIS FROM 



BEFORE. (Arnold.) Magnified 2| times. 



A, choroid ; B, iris ; c, ciliary muscle ; 

 1,1, long ciliary arteries ; 2, 2, five of the 

 antei'ior ciliary arteries ramifying at the 

 outer margin of the iris ; 3, loop of com- 

 munication between one of the anterior and 

 one of the long ciliary arteries ; 4, internal 

 circle and network of the vessels of the iris ; 

 5, external radial network of vessels. 



divides into an upper and a lower 

 branch, and these, anastomosing 

 with the corresponding vessels on 

 the opposite side and with the an- 

 terior ciliary, form a vascular ring 

 (circulus major, fig. 33, p, fig. 41 , 3) 

 in this situation. From this circle 

 small branches arise to supply the 



ciliary muscle ; whilst others converge towards the pupil, and there, freely com- 

 municating by transverse offsets from one to another, form a second circle of 

 anastomosis (circulus minor}, from which capillaries are continued inwardly and end 

 in small veins. 



The anterior ciliary arteries (fig. 33, c, fig. 41, 2, 2), five or six in number, 

 but smaller than the vessels just described, are supplied from the muscluar and 

 lachrymal branches of the ophthalmic artery, and pierce the sclerotic about 



VOL. III., PT. 3. D 



