650 



UOMPAKATIVE PHYSIOLOGY. 



not in structure unlike the parotid, the secretion of which its 

 own somewhat resembles. 



The saltness of the tears, owing to abundance of sodium 

 chloride, is well known to all. The nervous mechanism, of se- 

 cretion of tears is usually reflex, the stimulus coming from the 

 action of the air against the eyeball or from partial desiccation 

 owing to evaporation. When the eyeball itself, or the nose, is 

 irritated, the afferent nerves are the branches of the fifth, to 

 which also belong the efferent nerves. The latter include also 

 the cervical sympathetic. But it wQl, of course, be understood 

 that the afferent impulses may be derived through a large num- 

 ber of nerves, and that the secreting center may be acted upon 

 directly by the cerebrum (emotions). The excess of lachrymal 

 secretion is carried away by the nasal duct into which the lach- 

 rymal canals empty. While it is well known that closure of the 

 lids by the orbicularis muscle favors the removal of the fluid, the 

 method by which the latter is accomplished is not agreed upon. 

 Some believe that the closure of the lids forces the fluid on 

 through the tubes, when they suck in a fresh quantity ; others that 

 the orbicularis drives the fluid directly through the tubes, kept 

 open by muscular arrangements ; and there are several other 

 divergent opinions. The prevention of winking leads to irrita- 

 tion of the eye, which may assume a serious character, so that 

 the obvious use of the secretion of tears 

 is to keep the eye both moist and clean. 

 Though rudimentary in man, there 

 is in all our domestic animals a third 

 eyelid (membrana nictitans) which 

 may be made to sweep over the eye 

 and thus cleanse it. It is especially 

 well developed in those groups of 

 mammals that can not derive assist- 

 ance in wiping the eyes from their fore- 

 limbs, hence is found in perfection in 

 solipeds and ruminants. It is made up 

 of a fibro - cartilage, prismatic at its 

 base, and thus anteriorly where it is 

 ^'"lactaymar sartna" nasai Covered by the conjunctiva. It is most 

 tSriStTXrXpptyr ^^^^^^^ ^^ t^e inner canthus of the 

 eye, from which region it can spread 

 over the whole globe anteriorly. The flbro-cartilage is con- 

 tinued backward by a fatty cushion which is loosely attached 



