Via. 



ilia: Nervous System 



685 



OCULOMOTOR > 



Fig. 511. Diagrams of the eye-muscles of man. (A) The left eyeball and asso- 

 ciated muscles, viewed from the outer side. (B) The left eyehall with associated 

 muscles and nerves, viewed from behind. (After Warren and Carmichael: " Ele- 

 ments of Human Psychology," Boston, Houghton Mifflin Co.) 



(origin) in close relation to that of the rectus group. The loop (called 

 ""trochlea," meaning a wheel) is mechanically a pulley. Contraction 

 of the muscle pulls the long tendon backward but the pull of the 

 tendon on the eyeball is obliquely forward. This curious arrangement 

 of the dorsal oblique muscle is definitely a mammalian characteristic. 

 The retractor bulbi, a muscle found in amphibians and well 

 developed in reptiles, has origin on the wall of the orbit in close relation 

 to the origin of the muscles of the rectus group, and inserts on the 

 eyeball near the entrance of the optic nerve. It serves to draw the 

 eyeball back more deeply into the orbit — a protective measure. It is 

 commonly present in mammals, but often reduced or even quite 

 absent, especially in the "higher" groups. It is well developed in 

 ungulates, not found in whales, and occurs in man only as an occasional 

 "abnormality." 



Of the upper and lower eyelids (palpebrae), the upper is usually 

 wider and does the major part in closing the eye. Their outer surfaces 

 are usually covered by fine hair. The lid is commonly reinforced by a 

 thin plate, the tarsus, consisting of connective tissue or cartilage. The 

 edge of a lid is fringed by eyelashes, cilia, better developed on the upper 

 lid. Elephants and whales lack cilia. In close relation to the roots of 

 the cilia open the ducts of small ciliary glands which resemble sweat- 

 glands. Opening along the edges of the lids are the ducts of large com- 

 plex tarsal (Meibomian) glands which resemble sebaceous glands 

 in structure and produce an oily secretion (Figs. 512, 513). 



A nictitating membrane ("third eyelid") is usually present. It 

 is a fold of the very thin layer of integument, the conjunctiva, which 

 covers the inside surfaces of the upper and lower lids and continues 

 over the cornea. The fold may be at the anterior angle between upper 

 and lower lid, or it may be mainly within the lower lid. It is very thin, 

 hairless, and transparent. In its degree of development it ranges from 

 a maximum in the common cat, in which it may be drawn completely 



