SENSE ORGANS 



537 



stratified epithelium and internally by pavement epithelium. 

 It has nerves, but no blood vessels or lymphatics. The choroid, 

 which is richly pigmented, has three layers corresponding to 

 the dura mater, arachnoid and pia mater. From it are formed the 

 intrinsic muscles of the eye. The lens has an elastic capsule, 

 and lens fibres formed from epithelium. 



Future retina 



Future pigment layer 



Outer wall of 

 optic vesicle 



Inner invaginated 

 wall of optic 

 vesicle 



Inner surface 

 of optic cup 



Cavity of 

 neural tube 



Fig. 420. — A vertical section of the developing optic cup, showing how the 

 sensitive cells of the retina come to lie on the outer surface. — From Willmer. 

 (Bourne, Cytology and Cell Physiology, 2nd Edition, 195 1. Clarendon Press, 

 Oxford.) 



BLOOD VESSELS 



Arteries (Fig. 421) are lined by endothelium, consisting of flat- 

 tened scale-Hke cells ; it is surrounded by a httle elastic tissue, and 

 outside this again is a thick coat of elastic tissue and plain 

 muscle, and outside this again is collagenous tissue. The proportion 

 of muscle to elastic tissue varies, and in general the larger the 

 vessel the higher the proportion of elastic fibre. The great 

 vessels near the heart are therefore able to take up the increased 

 pressure caused by systole by an expansion followed by an 

 elastic recoil-contraction. These appear as the pulse. In the 

 finest arteries, the arterioles, there is nothing but muscle in the 

 middle coat, and in the capillaries all the parts of the wall have 

 gone except the endothelium ; the transition from the great 

 M.z. — 18 



