OLFACTORY ORGAN 



203 



teriorly to it. This turbinal is comparable to that of Urodeles and 

 Reptiles. 



The so-called external nasal gland of Birds is situated on the 

 frontal or nasal bones, along the upper margin of the orbit. It 

 is supplied by the first and second branches 

 of the trigeminal, and corresponds to the 

 lateral nasal gland of Lizards. 



Mammals. Corresponding to the much 

 more marked development of the facial por- 

 tion of the skull, the nasal cavity of Mammals 

 is proportionately much larger than in the 

 forms yet described, and consequently there 

 is much more room for the extension of the 

 turbinals : these give rise to a spongy laby- 

 rinth, the cell-like compartments of which 

 .are lined by mucous membrane ; and thus 

 variously shaped projections, supported partly 

 by cartilage and partly by bone, are seen ex- 

 tending into the nasal cavity (Fig. 163, A c). 



The normal number of these true olfactory 

 ridges or scrolls varies considerably. 1 They 

 may be arranged in one row (Orni thorny n- 

 chus, Cetacea, Pinnipedia, Primates), or in 

 several rows (other Mammals), in which latter 

 case the olfactory lobes are largely developed. 



According to the degree of development of the olfactory appa- 

 ratus, taking specially into account its cerebral portion (olfactory 

 lobes), we may distinguish between Mammals which are macros- 

 matic (the majority of the mammalian orders), microsmatic (Seals, 

 Whalebone Whales, Monkeys, Man), and anosmatic (most Toothed 

 Whales). 



The above-mentioned olfactory scrolls belong to the true olfac- 

 tory region, and are generally described as " ethmoid turbinals" 

 as in all but the first of the series their skeletal supports usually 

 become united with the ethmoid bone, the first coming into rela- 

 tion with the nasal, and being therefore usually spoken of as the 

 " nasal turbinal" It must, however, be borne in mind that these 

 do not correspond to the turbinals of lower Vertebrates. The latter 

 -are represented by the so-called " maxillary turbinal" situated in 

 the anterior (lower) portion of each nasal chamber, which com- 

 municates with the pharynx by the internal nostrils, its skeletal 

 portion becoming united with the maxillary bone (Fig. 163, c). 

 This maxillary turbinal no longer possesses an olfactory epithelium 



1 In most of the Mammalian orders, five olfactory scrolls are typically present ; 

 in Echidna there are six or more ; in Ungulates there may be as many as 

 eight ; amongst Edentates, Orycteropus possesses eleven ; while in adult 

 Primates there are only from one to three, a greater number being present in the 

 mbryo (Fig. 163). 



FIG. 162. TRANSVERSE 

 SECTION THROUGH THE 

 RIGHT NASAL CAVITY 

 OF A SHRIKE (Laniu* 

 minor. ) 



OM, MM, superior 

 (pseudo) and middle 

 (true) turbinal ; a, 

 upper, and b, lower 

 nasal passage ; LR, 

 air - chamber, which 

 extends into a hollow 

 of the superior tur- 

 binal. 



