. The chordates. Chordata. 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 te
. The chordates. Chordata. 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 "; 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.
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