266 COMPARATIVE ANATOMY 



The structure of the nose in Chelonians is very complicated 

 and varied. In marine Chelonians the organ is divided into two 

 passages, one above the other, and connected by means of a per- 

 foration of the septum. The comparative paucity of glands in 

 the olfactory organ of Lizards and Snakes forms a marked 

 contrast to the condition seen in Chelonians, the nasal organ of 

 which is characterised by a great abundance of them. 



The extension downwards and backwards of the olfactory organ 

 is most marked in Crocodiles, in correspondence with the forward 

 growth of the facial region and the formation of the palate ; its 

 posterior part thus lies below the brain and base of the skull, the 

 naso-pharyngeal passage being so much elongated that the 

 posterior nostrils open far backwards into the pharynx. Each 

 nasal chamber is divided posteriorly into two superimposed cavities, 

 the upper of which represents the proper olfactory chamber, and is 

 lined by sensory epithelium, while the lower serves as a respiratory 

 passage only. Certain accessory air-chambers are connected with 

 the nasal cavity. A large gland is present between the olfactory 

 chamber and its investing bones, and opens on either side of the 

 nasal septum, posteriorly to the external nostrils, by one or two 

 apertures. As in other Reptiles, there is only a single true 



turbinal, but externally to it lies a second 

 0]ir prominence, which may be spoken of as a 



pseudo-turbinal, and which possibly corre- 

 sponds to the upper turbinal of Birds. 



Birds. In all Birds, as in Lizards, there 

 is an outer chamber lined by stratified epi- 

 thelium, and an olfactory chamber proper, 

 situated above the former. In addition to 

 FIG. 195. TRANSVERSE a turbinal corresponding to that of Reptiles 

 SECTION THRO UGH THE and usually known as the middle turbinal, 

 RIGHT NASAL CAVITY th j so-called upper turbinal (Fig. 195) : 

 OF A SHRIKE (Lnnuix . -i i ji -11 



minor). the former is comparable to the maxillo- 



turbinal and the latter to the naso-turbinal 



n, upper, and o. lower ,, , . , 



nasal passage; LK, of Mammals (q.v.). A special projection 

 air-chamber, which composed of undifferentiated epithelium and 

 extends int.. a hollow grated i n the outer chamber may be dis- 



ot the upper turbinal; . . , , ., , , 7 . ; r m 



OAf, MM, upper and tmguished as the vcstibular turbinal. Inere 

 middle turbinals. is no longer any olfactory epithelium on 



the middle turbinal in the adult, and the 



upper turbinal during development gradually passes backwards 

 relatively to the middle turbinal, which is usually supported by 

 cartilage or more rarely by bone, and the form of which varies 

 greatly. It may be represented by a moderate-sized prominence, 

 or may become more or less rolled on itself: the lacrymal duct 

 opens below and anteriorly to it. The narrow, slit-like internal 

 nostrils open comparatively far back. 



