Practicum VII. The Muscles of the Eye. 45 



with the other hole and note its emergence upon the other lid. These' 

 are the CANALICULI or lachrymal canals. 



a. Were the adjacent parts retained these two canals would be found 

 to open into a sack, the LACHRYMAL SACK, continuous through the nasal 

 duct into the cavity of the nose. 



b. If the cat's head has been retained these parts may be traced, 

 though with some difficulty on account of their small size. The ventral 

 end of the nasal duct is just laterad of the ventral turbinal, maxillo-tur- 

 binal, indicated in PI. XVII by the abbreviation Mxtrb. 



c. Through the passages above described the natural moisture of the eye, keeping 

 the apposed conjunctival services soft, is drained away into the nose and evaporated by the 

 currents of air. Where there is an excess of lachrymal secretion, as from taking cold, 

 from the odor of an onion, or from laughter or crying, part runs over the edges of the lids 

 as tears, and part makes itself apparent in the nasal cavity. 



d. The orifices of the canaliculi at the margins of the lids are narrow and valvular so 

 that particles of any size do not readily enter. 



e. The small curved blunt pointed knife called by anatomists syringotome (tube- 

 opener) is used by surgeons in opening up the lachrymal canals, and is called by them 

 canaliculus knife. 



f. The lids may be cut away with the scissors. 



# 12. The Cornea and Solera. On the free surface of the ball the elliptical area in- 

 cludes the CORNEA, transparent during life but rendered opaque by alcohol ; the rest of 

 the surface of the ball is constituted by the naturally white and opaqe SCLERA, commonly 

 called Sclerotic. 



a. In the natural attitude of the sheep's head the long axis of the cornea is nearly 

 parallel with that of the brain but not with that of the head as a whole. 



13. The Caudal Aspect. Note again the following features more 

 in detail than above (3). 



a. FAT and CONNECTIVE TISSUE, in white, irregular masses ; they 

 constitute a cushion for the ball ; during prolonged illness or fasting the 

 fat wastes so that the eye literally becomes "sunken." 



b. In the midst, the white, firm, fibrous OPTIC NERVE. 



c. The cut ends of the MUSCLES. 



14. The Rectus Muscles. With the fingers (using the knife-point 

 or tracer sparingly), pulling mostly from the center, tear out the fat and 

 connective tissue so as to separate four muscles, at the four opposite sides 

 of the ball ; these are the RECTI (straight) muscles. Their TENDONS unite 

 to form a continuous sheet, thin but tough, entad of the severed con- 

 junctiva. 



a. There are two OBLIQUE muscles whose location and attachments 

 are less easily recognized ; see the models and diagrams. 



b. The superior oblique muscle passes through a fibrous loop at the 

 mesal side of the orbit as through a pulley, and may be exposed if time 

 permits. 



c. Immediately surrounding the optic nerve is another layer of 

 muscle, but it does not exist in man and may be disregarded. 



15. Demonstration of the Actions of the Rectus Muscles. Hold the 

 eye lightly between the left index and pollex, the dorsal side up. Grasp 

 the rectus that is attached to the mesal side of the ball, i. e., correspond- 

 ing with the end of the cornea which is partly covered by the plica. Pull 

 the muscle and thus rotate the ball mesad, i. e., inward. Pull the muscle 

 at the opposite side and rotate it laterad, i. e., outward 



a. Strabismus or "squint" may be due to either the shortness or 

 undue contraction of one muscle, or to the length or weakness of its an- 

 tagonist. 



b. With the scissors trim off the rectus muscle and the plica. 



