Biology of Man 497 



Photoreceptors (receptors sensitive to light) 

 In the human eye (Figs. 248 and 249) hght waves pass through the 

 transparent cornea, through the aqueous humor, and the pupil 

 (opening in the colored iris), striking the lens. The latter changes 

 its shape, thereby focusing the light and producing an image on the 

 rods and cones (sensitive cells) of the retina. This starts a photo- 

 chemical change in the photosensitive visual purple located in the 

 cells of the retina. This change induces action currents in the optic 

 nerve which carries them to the brain (Fig. 247) . 



-«jn/j rectus niu^'cle 



Sclera.- - -y^. 



Choroid---/ 

 Reitna /- 



ConjuncttVcL 

 / 



chamber 



VA // Posterior 

 chuuiber 



x,y Ctlictrij body 



■'In f. rectus muscle 



Fig. 248.- — Human eyeball in vertical section shown diagrammatically. The 

 anterior and posterior chambers contain aqueous humor; the vitreous body is a 

 transparent jellylike substance; the circular opening in the iris is the pupil; the 

 ciliary body is composed of bundles of smooth muscles to control the lens; the 

 inner layer (retina) is composed of light-sensitive nerve cells. (From Francis: 

 Introduction to Human Anatomy, The C. V. Mosby Co.) 



4. Thermoreceptors (receptors sensitive to heat) 



These are located in the skin in various parts of the body. 



5. Osmotic receptors (thirst) located in the mouth and throat 



6. Pain receptors — free nerve endings of sensory nerves located in various 



parts of the body. 



The effectors in man are mainly glandular (for secretion) and me- 

 chanical (for movements). Secretions are produced by the protoplasm 

 of cells from the food materials brought to them. In man there may be 

 isolated secretory cells or groups of secretory cells associated together to 



