312 ELEMENTARY ANATOMY. [LESS. 



The eye muscles may attain a prodigious length, as in the 

 Hammer-headed Shark, where they arise from the basis 

 cranii and extend the whole length of the prolonged lateral 

 processes which support the eyeballs at their ends. 



There may be a distinct depressor of the lower eyelid, and 

 in addition to this, other muscles may be developed in con- 

 nexion with the third eyelid, which is of large size in so many 

 animals, notably in Birds. 



The third eyelid may be furnished with a muscle which, 

 arising from the temporal side of the orbit, passes through 

 a muscular and ligamentous loop, and is inserted into the 

 third eyelid's inferior margin. 



This muscle may be, as in the Frog, furnished with a tendon 

 which forms' a loop over the conical muscular mass of the 

 eyeball, so that when that muscle swells by the retraction of the 

 eyes the loop necessarily contracts, and thus moves the eyelid. 



It may be, as in the Crocodile, that the muscle of the third 

 eyelid takes origin from the upper and inner part of the eye- 

 ball, and, passing downwards round the conical muscle and 

 optic nerve, reaches the lower angle of the third eyelid. 



Finally, as is the case in Birds, it may spring from the 

 lower, nasal side of the eyeball and end in a tendon which 

 proceeds to its insertion as in the Crocodile, except that 

 it passes through a pulley formed by the tendinous sheath of 

 a second muscle (the quadratics nictitantis) which arises from 

 the sclerotic at its upper and back part, and ends in forming 

 the sheath aforesaid. As in Birds there is no conical muscle, 

 this quadrate one is probably its representative. 



Another muscle may also exist, as in the Frog, forming a 

 sort of fleshy sheet on which the eyeball rests, and which 

 protrudes the eyeball by its contraction. 



I/. As to the MUSCLES OF THE NECK. 



The sterno-cleido-mastoid of man really represents what 

 are two muscles in many other animals, viz. a sterno-mastoid 

 and a cleido-mastoid. Nevertheless it may be a single 

 muscle even in Reptiles, as in the Iguana. 



The sterno-mastoid may be wanting altogether, as in 

 Birds and Batrachians. 



The cleido-mastoid may also be wanting, and it may, as in 

 the Horse (by suppression of the clavicle and uninterrupted 

 union with the deltoid or pectoralis), extend directly from the 

 skull to the humerus as a cephalo-humeral, or even to the 

 ulna, as in Hyrax. 



In cold-blooded Vertebrates this muscle maybe attached to 



