210 Anatomy and Physiology of the Visual Oigafis. 



pair ; another branch of this fifth pair is distributed on the 

 Schneiderian membrane, which lines the nostril; but the first 

 pair, or olfactory (oleo, to smell, and Jacio, to make) nerve, is 

 spread upon this membrane; hence the sympathy above 

 noticed. And again, a branch of the fifth pair unttes with 

 a branch of the sixth, and these by their vmion form the 

 great intercostal nerve {inter-, between, costa, rib) ; but some 

 twigs of this join the eighth pair, which is spread on the lungs 

 and adjoining parts ; and hence a strong odour will cause 

 coughing also. The other defences of the eye are the lids (^), 

 which by their quick motion protect the organ, without im- 

 peding the pencils of light from entering the pupil, and spread 

 the secretion from the lacrymal gland over the cornea. Each 

 lid is armed with a row of hairs, which, when the eyes are 

 closed in sleep, form an impenetrable palisade against the 

 most minute dust or insect. The lids are lined with a very 

 delicate membrane, which is reflected over the front of the 

 eyeball, and is named the conjunctiva (conjungo, to unite). 



In man there may be seen a little crescent-shaped fold of 

 the conjunctiva in the inner corner of the eye, called the 

 pUca lundris {sl fold, in shape like a half moon): in birds 

 and horses this is more fully developed, and is of essential 

 benefit to those animals. This seems to be one of those 

 highly interesting phenomena in the works of nature, where 

 in one race of beings there are only rudiments of organs, to 

 all appearance totally useless, unless it be to show a beautiful 

 uniformity of design pervading all her works ; whilst in others 

 of opposite habits they are more complete, and indispensable 

 to the health of the creature. Thus, in our own species, the 

 male has the rudiments of mammae, totally unserviceable to 

 himself or others, whilst in the female they are positively 

 necessary to the nourishment of her offspring. In human 

 beings, again, the ccecum (or blind gut) is extremely small ; 

 in horses it is more developed, and also in other graminivo- 

 rous animals ; and in fishes the coeca are often seen very 

 numerous and long, being collected together in a cluster, and 

 wearing the appearance of a thick tassel. Analogous to these 

 is the plica lunaris, which in man seems merely an unservice- 

 able rudiment of the tunica nictitans in birds and horses. 

 To them it serves as a third eyelid, which can be drawn over 

 and relaxed at the will of the animal by a simple but elegant 

 mechanism, not to be found in the human subject.* Lastly, 

 the little red tubercle in the inner corner of the eyelid, termed 



* For a more exact description and diagram of this apparatus, see a 

 " Discourse on the Objects, Advantages, and Pleasures of' Science, with 

 Engravings," 8vo, Baldwin, London; by Mr. (now Lord) Brougham. 



