GROANS OF SENSE. 705 



we shall find to correspond most exactly in all essential points through- 

 out the different classes of Vertebrata. 



(1979.) The olfactory nerves leave the olfactory lobes of the brain 

 as single round cords, and are not, as in the Mammalia, divided into 

 numerous filaments ; consequently there is no cribriform plate to the 

 ethmoid bone, but the nerve of each side (fig. 350, e) is received into a 

 simple canal, partly osseous and partly cartilaginous, through which it 

 is conducted to the cavity of the nose. 



(1980.) The nasal apparatus of .Reptiles differs from that of Eishes 

 in one important particular. Breathing air as these creatures do, the 

 sense of smell now becomes connected with the respiratory function ; 

 and a communication being established between the nasal cavities and 

 the larynx, the air which passes through this channel into the lungs 

 must necessarily come in contact with the sentient surface formed by 

 those portions of the lining membrane of the nose to which the nerves 

 of smell are distributed ; and in proportion as the extent of that surface 

 becomes developed, the power of appreciating the presence of odorous 

 particles in the atmosphere will necessarily be increased. The physio- 

 logist is thus enabled to estimate with great exactness the relative per- 

 fection of the sense of smell in different classes, or even in different 

 families of the air-breathing Yertebrata, simply by observing the com- 

 plication and extent of surface presented by the lining membrane of the 

 olfactory organ. 



(1981.) Taking this as our guide, we must suppose that in all rep- 

 tiles the sense in question is extremely obtuse, since in these creatures 

 there are neither turbinated bones nor ethmoidal plates as yet distin- 

 guishable, a few folds of the membrane lining the nose, even in those 

 species which are most highly gifted in this particular, being the only 

 provision for extending the olfactory surface ; and in many cases, as for 

 example in the Amphibia, the nose seems merely a simple canal leading 

 into the mouth. 



(1982.) On reaching the nasal cavity, the olfactory nerve spreads 

 out into delicate filaments (fig. 350, d), which are distributed to the 

 Schneiderean membrane covering the septum and upper part of the nose. 



(1983.) The optic nerves of Keptiles (fig. 348, n), soon after their 

 origin, become confounded together by a commissure, in the same way 

 as in the human subject; and again separating, they are continued 

 through the optic foramina to the eyes. 



(1984.) The eyeball itself presents few peculiarities in its structure. 

 In the Tortoise, and many Lizards, the sclerotic contains a circle of bony 

 plates imbedded in its substance, and surrounding its anterior margin : 

 these are obviously the rudiments of that osseous zone which in the 

 class of Birds, as we shall find, performs a very important office. The 

 ciliary processes of the choroid are generally very feebly developed. 

 The pupil is frequently round; but it is sometimes of a rhomboidal 



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