1312 TEARS. [BOOK in. 



layers only, the upper cells being somewhat flattened and the 

 dermis being thrown up into scattered papillae. 



Imbedded in the tarsus, stretching from the hind border to 

 the free edge of the lid lies, in each eyelid, a row of thirty or 

 fewer largely developed sebaceous glands (437) the Meibomian 

 glands. Sebaceous glands are also attached to the follicles of the 

 eyelashes, and into the ducts of some of these open the glands of 

 Moll, which have the structure of a sweat gland ( 436). Small 

 mucous glands are moreover found in the conjunctiva especially 

 in the neighbourhood of the fornix. 



These several glands contribute to keep the surface of the eye 

 and eyelids moist ; but this is chiefly effected by the secretion of 

 the lachrymal gland which is placed above the upper eyelid in the 

 lateral region of the orbit, and which, imperfectly divided by an 

 extension of the tendon of the levator palpebrarum into two 

 masses, discharges its secretion by several ducts opening along the 

 fornix conjunctivas. Under ordinary circumstances the fluid thus 

 secreted is carried away through the punctum lachrymale of the 

 upper and of the lower eyelid, at the inner angle of the eye, into 

 the lachrymal canaliculi, and so into the lachrymal sac, and finally 

 into the cavity of the nose. When the secretion becomes too 

 abundant to escape in this way it overflows on to the cheeks in 

 the form of tears. 



The structure of the lachrymal gland is in its main features 

 identical with that of an albuminous salivary gland, or with that 

 of the parotid, save that the epithelium of the ducts is never 

 striated ; it will be unnecessary to describe it in detail. In some 

 animals a somewhat peculiar gland, the Harderian gland, lies in 

 the inner (median) region of the orbit ; this varies in structure in 

 different animals, being in some a sebaceous gland united with a 

 gland similar in structure to the lachrymal gland. 



If a quantity of tears be collected, they are found to form 

 a clear faintly alkaline fluid, in many respects like saliva, contain- 

 ing about 1 p.c. of solids, of which a small part is proteid in nature. 

 Among the salts present sodium chloride is conspicuous. 



The nervous mechanism of the secretion of tears, in many 

 respects, resembles that of the secretion of saliva. A flow is usually 

 brought about either in a reflex manner by stimuli applied to the 

 conjunctiva, the nasal mucous membrane, the tongue, and the 

 interior of the mouth, or more directly by emotions. Powerful 

 stimulation of the retina by light will also cause a flow, as will 

 electrical or other stimulation of any of the cranial or upper spinal 

 afferent nerves. Venous congestion of the head is also said to 

 cause a flow. The efferent nerves are the lachrymal and orbital 

 branches of the fifth nerve, especially the former, stimulation of 

 these causing a copious flow. It is said that stimulation of the 

 cervical sympathetic will also cause a somewhat scanty flow of 

 turbid tears, but on this point all observers are not agreed. 



