'2-2 THE CHOROID. [BOOK in. 



It consists on the one hand of liranehed or irregularly polygonal 

 corpuscles, generally loaded with pigment in the form nf minute 

 ellipsoidal cry >t a Is. and on the other hand of elastic fibres of 

 various degrees of fineness, both elements being imbedded in a 

 homogeneous ground substance, and for the most part arranged 

 in lamellae. The choroid coat therefore in contrast to the sclerotic 

 coat is a conspicuously elastic coat. 



The coat as a whole may be divided into three or more layers 

 lying one above the other. 



Immediately beneath the sclerotic the coat is loose in texture, 

 and is made up of a series of lamella* four or five in number, sepa- 

 rated by lymph-spaces lined with epithelioid plates. Each lamella 

 consists, as just described, of pigment cells and elastic fibres 

 imbedded in a ground substance. This part of the coat, which is 

 often called the suprachoroidal membrane, is separated by large 

 irregular lymph-spaces from the inner surface of the sclerotic which 

 being rich in pigment cells and of somewhat looser texture than 

 the rest of the sclerotic is sometimes distinguished as the lamina 

 fusca. 



Below the suprachoroidal membrane lies the layer containing 

 the larger blood vessels, often spoken of as the choroid proper. 

 These blood vessels are on the one hand the trunks of the ciliary 

 arteries, short and long, with the branches into which these break 

 up, and on the other hand the ciliary veins, which gathered up from 

 the ciliary processes and iris as well as from the choroid itself, are 

 arranged along the equator of the eyeball in a number of venous 

 whorls, the vena.' vorticoscc, the issuing veins of which pierce the 

 sclerotic. In a vertical section through a prepared and hardened 

 choroid these larger veins and arteries are seen cut through in 

 various ways, the relatively small spaces between them being 

 occupied by pigment cells and elastic fibres ; towards the inner 

 surface the fibres are especially abundant and the pigment cells 

 scanty. The blood vessels are surrounded by perivascular lymph- 

 spaces and the tissue between the vessels contains numerous small 

 lymph-spaces lined with epithelioid plates. 



Beneath this layer of arteries and veins lies another layer con- 

 sisting almost entirely of capillaries into which the arteries break 

 up and from which the veins are gathered up. This is called the 

 chorio-capillary membrane. The capillary network is exceedingly 

 close set, almost as close as that of the pulmonary alveoli ; and the 

 tissue between the vessels, reduced to a minimum, consists of a 

 homogeneous or finely dotted ground substance in which a few 

 branched cells, devoid of pigment, may occasionally be seen, as 

 well as, especially in myopic eyes, wandering leucocytes. Peri- 

 vascular lymph-spaces are said to surround the capillaries. 



This chorio-capillary membrane rests on a transparent homo- 

 geneous membrane, about 2/* thick, called the membrane of 

 Bruch, or basal membrane, which separates the choroid from the 



