DISSECTION OF THE NECK. 61 



The canal in the upper eyelid is longer and more arched than that in the 

 lower lid. 



The lachrymal sac and duct (fig. 13) extend from the inner part of the 

 orbit to the nose, and convey the tears into the latter cavity. They form 

 one tube, of which the upper dilated part is the sac, and the lower con- 

 stricted end the duct. 



The sac, 5 , is situate in the hollow formed by the os unguis and nasal 

 process of the superior maxillary bone. Externally it is crossed by the 

 ligament of the eyelids, and is covered by an expansion derived from that 

 band, which is fixed to the margin of the bony groove. If the aponeurotic 

 covering be removed, the mucous membrane lining the interior will be 

 seen. Into the outer side of the sac the lachrymal canals open. 



The duct, 6 (ductus ad nasum), is the narrowed part of the tube, and is 

 about half an inch long. It is entirely encased by bone, and corresponds 

 with the passage of the same name in the dried skull. In the nasal cavity 

 it opens into the front of the inferior meatus, where its opening is guarded 

 by a piece of the mucous membrane. A bent probe introduced through 

 the nostril may be passed into the duct from the meatus, but with destruc- 

 tion of the valve. 



As the duct is continuous with the mucous membrane of the nose it has 

 the same structure, viz., a fibrous external layer with a mucous lining. The 

 epithelium of the sac and duct is ciliated as in the nose, but in the lachry- 

 mal canals it is scaly. 



Directions. The examination of the eyeball may be omitted with more 

 advantage to the student till the dissection of the head and neck has been 

 completed. The description of the eye will be found at the end of the 

 book. 



SECTION V. 



DISSECTION OF THE NECK. 



Position. For the dissection of the right side of the neck let the head 

 be supported at a moderate height on a block, and let the face be turned 

 to the left side and fastened in that position with hooks. To obtain a good 

 view of the region, the right arm may be drawn under the body, with the 

 object of depressing the point of the shoulder, and putting the neck parts 

 on the stretch. In some bodies, owing to a difference in the form of the 

 neck, the best position will be obtained by placing the upper limb over 

 the chest. 



Surface-marking. The side of the neck presents a somewhat square out- 

 line, and is limited in the following way : Below is the prominence of 

 the clavicle; and above is the base of the lower jaw with the skull. In 

 front the boundary is a line from the chin to the sternum ; and behind, 

 another line from the occiput to the acromial end of the clavicle. The 

 part thus included is divided into two triangular spaces (anterior and pos- 

 terior) by the diagonal line of the projecting sterno-mastoid muscle. And 

 in consequence of the position of that muscle the base of the anterior space 

 is at the jaw, and the apex at the sternum ; whilst the base of the posterior 

 one is at the clavicle, and the apex at the head. 



The surface in front of the sterno-mastoid is depressed at the upper part 



