VAN 121: Veterinary Gross Anatomy-II (2+2)

CHORIOID

• It is a thin membrane lying between the sclera and retina.
• It is loosely attached to the sclera by the lamina fusca except where vessels and nerves pass through.
• The internal surface is in intimate contact with the pigmentary layer of the retina.
• The chorioid is generally dark brown in colour but an extensive area above the level of the optic papilla has a metallic lustre, the colour varying from iridescent blue to green called the tapetum of the chorioid. Posteriorly it is perforated by the optic nerve.
• It is continuous anteriorly with the ciliary body.
• The tissue of the chorioid is principally made up of blood vessels with numerous pigment cells.
• It consists of the following layers,
  • (Lamina suprachoroidea of fibrous tissue with elastic fibres containing pigment cells
  • Lamina vasculosa of large blood vessels
  • Lamina choriocapillaris between this and the previous layer is a network of fibroelastic tissue, the tapetum fibrosum which gives the metallic lustre and it is here only the chorioid coat is devoid of pigment cells and blood vessels. Tapetum fibrosum is built up of bundles of laminated areolar connective tissue cells superimposed upon the chorioid to form a tendinous type of surface that reflects back the light passing through retina. This causes tapetum to glisten and exhibit various types of coloration according to species and age of the animal; (the tapetum is absent in man, monkey and pig)
  • Lamina basalis is thin and transparent and is made up of an inner homogeneous and an outer elastic layer.