ANATOMY IN A NUTSHELL. 389 



The eyebrows (supercilia) are two arched eminences over each orbitc,on- 

 sisting of thickened integument and muscles and are covered by thick hairs. 

 Each one is connected with the Orbicularis palpebrarum, Corrugator super- 

 cilii, and Occipito-frontalis. 



Eyelids. The upper one is more movable than the lower one. When 

 closed they protect the eye-ball from injury. Where the upper and lower lids 

 meet when open they form the outer and inner canthus. The lacus lachrymaiis 

 is a small triangular area in the inner canthus in which is a mass of fat (pink) 

 connective tissue. This mass is caruncula lachrymaiis and it is separated from 

 the eye-ball by a vertical fold of conjunctiva, the plica semilunaris which is a 

 rudimentary third eyelid. 



The eyelashes (cilia) are short, thick hairs which form a double row at the 

 free margin of each lid. Those of the upper lid are longer and more numerous 

 than those of the lower lid. Each eyelid has the following structures from with- 

 out inward: (1) Skin, (2) areolar tissue, (3) Orbicular muscle, (4) tarsal 

 plate, (5) palpebral ligament, (6) Meibomian glands, (7) conjunctiva. The 

 upper lid has in addition the aponeurosis of the Levator palpebral which is at- 

 tached to the upper surface of the tarsal plates. 



The tarsal plates are in each lid and are formed by lamina of a dense con- 

 nective tissue. The upper one is the larger; it is half oval in shape while the 

 lower one is a narrow oblong slit. 



The Meibomian glands are lodged in these plates. The palpebral ligaments 

 are at the outer and inner angle of the eye, forming attachments for the Orbi- 

 cularis palpebrarum muscle. They form an imperfect diaphragm for the anterior 

 orifice of the orbit. Peripherally they are attached to the orbital margin and 

 centrally to the edges of the tarsal plates. (See Plate CCXII.) 



The third or motor oculi has its superficial origin in front of the pons on 

 the inner side of the crus cerebri. Its deep origin is situated on each side of the 

 median line on the floor of the aqueduct of Sylvius, after passing through the 

 substantia nigra and the tegmentum of the crus. All the muscles of the orbital 

 region are supplied by the third nerve except the Superior oblique, which re- 

 ceives the fourth nerve and the External rectus, which receives the sixth. This 

 nerve also supplies the sphincters of the iris and the ciliary muscle through its 

 connection with the ciliary ganglion. Remember that the External rectus of 

 one eye and the Internal rectus of the other eye have their nerve supply from 

 the same nucleus. The nucleus of the third nerve receives the fibers from that 

 of the sixth nerve of the opposite side; this accounts for the fact that one can 

 move both eyes to the right or left at the same time. From this deep origin 

 of the third nerve each muscle supplied by it has its own set of cells. Then from 

 behind forward their position is (1) those cells to the Internal rectus; (2) to the 

 Levator palpebral; (3) the Superior rectus; (4) the Inferior rectus; ami (">) the 

 Inferior oblique. In front of these are the nuclei of the fibers for accommoda- 

 tion and for sphincter pupillae. It is covered with pia mater and arachnoid on 

 leaving the brain. It pierces the dura mater anterior and external to the pos- 

 terior clinoid process after having passed between the superior cerebellar and 

 the posterior cerebral arteries. It then passes through the cavernous sinus 



