978 TEARS. [BOOK in. 



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

 or fewer largely developed sebaceous glands the Meibomian 

 glands. Sebaceous glands are also attached to the follicles 

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



lands of Moll, which have the structure of a sweat gland, 

 mall 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 secre- 

 tion 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 palpebra- 

 rum into two masses, discharges its secretion by several ducts 

 opening along the fornix conjunctivas. Under ordinary circum- 

 stances 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. In some animals a somewhat peculiar gland, the Har- 

 derian 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, con- 

 taining about 1 p.c. of solids, of which a small part is pro- 

 teid 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. 



The chief use of the act of blinking is to keep the surface of 



