12 TEXT-BOOK OF ZOOLOGY 



(e) Limbs. These portions of the skeleton display the most variable 

 types of structure, according to the manner of locomotion of the animal 

 under consideration. They are, however, always composed of the 

 following parts : 



The fore-limb consists of the upper arm, or humerus ; the forearm, 

 made up of two bones, the radius and ulna, of which the latter is often 

 rudimentary ; and the hand, or manus, consisting of the carpus, meta- 

 carpus and digits (fingers). 



The hind-limb consists of the thigh-bone, or femur ; the leg proper 

 (crus), made up of two bones, tibia and fibula (the latter often rudi- 



VERTICAL SECTION THROUGH THE HUMAN EYE. 



wAu., Sclerotic ; H., cornea ; A., choroid ; R., iris ; P., pupil ; Sn., optic nerve ; N"., retina ; 

 L., lens ; Stk. and Stb., ciliary muscle and ciliary fold, organs for maintaining the lens in 

 a tense condition and for regulating the degree of its convexity ; G., vitreous body ; 

 B., the image, on the retina, of the object Pf. 



mentary) ; and the foot, or pes, consisting of the tarsus, metatarsus, and 

 digits (toes). The knee-joint is protected in front by a special bone, the 

 kneecap, or patella. 



3. Organs of Sense. (a) The eyes lie within protective cavities, the 

 orbits. The wall of the eyeball is formed of three membranes. The outer- 

 most of these, the sclerotic, serves as a protecting covering. In front 

 this covering is transparent like the purest glass, in order to permit the 

 entrance of light into the interior of the eye. This transparent portion 

 is termed the cornea. The second membranous layer is termed the 

 choroid, and is well supplied with bloodvessels (supplying nutriment to 

 the eye). From the circumference of the cornea the choroid stretches 

 across the eye like a curtain, which is known as the iris. In the centre 

 of the iris there is an aperture for the entrance of light, forming the 

 pupil. This aperture contracts in a bright light, and dilates in a faint 



