e502 THE SENSORIAL FUNCTIONS. 



particular attachment, but is employed for the 

 purpose of serving as a loop for the passage of 

 the long tendon of the second muscle (p), which 

 is called the pyramidalis, and which arises from 

 the lower and back part of the sclerotica. Its 

 tendon (t), after passing through the channel 

 above described, which has the effect of a pulley, 

 is conducted through a circular sheath, furnished 

 by the sclerotica to the under ^^vt of the eye, 

 and is inserted into the lower portion of the 

 loose edge of the nictitating membrane. By 

 the united action of these two muscles, the 

 former of which serves merely to guide the 

 tendon of the latter, and increase the velocity of 

 its action, the membrane is rapidly drawn over 

 the front of the globe. Its return to its former 

 position is effected simply by its own elasticity, 

 which is sufficient to bring it back to the inner 

 corner of the eye. If the membrane itself had 

 been furnished with muscular fibres for effecting 

 this motion, they would have interfered with its 

 use by obstructing the transmission of light. 



The eyes of quadrupeds agree in their general 

 structure with those of man. In almost all the 

 inferior tribes they are placed laterally in the 

 head, each having independent fields of vision, 

 and the two together commanding an extensive 

 portion of the whole sphere. This is the case 

 very generally among fishes, reptiles, and birds. 

 Some exceptions, indeed, occur in particular 



