176 COMPARATIVE ANATOMY. 



is no doubt that this turbinal is comparable to that of Urodeles 

 and "Reptiles. 



The so-called external nasal gland of Birds does not lie in the 

 region of the upper jaw, but on the frontal or nasal bones. 



Mammals. Corresponding to the much more marked de- 

 velopment of the facial portion 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 turbinal s : these give rise to a spongy labyrinth, 

 the cell-like compartments of which are lined by mucous membrane, 

 and thus variously shaped projections, supported partly by car- 

 tilage and partly by bone, are seen extending into the nasal 

 .-cavity. The normal number of these "olfactory scrolls" is 

 ^ five. In Marsupials they have a very typical arrangement ; they 

 are situated obliquely, slanting from the postero-dorsal to the 

 antero-ventral side : the inferior is no longer covered by olfactory 

 epithelium, and it becomes connected with the maxilla. 



The four other typical (ethmoidal) scrolls may persist as 

 such, or the two upper and two lower become united together, 

 in which case they are called respectively the superior and 

 middle turbinal s. Usually, however, the two upper primary 

 turbinals remain separate throughout life, so that in this case there 

 are two upper turbinals. The middle turbinal may also remain 

 partially or entirely separated into its two primitive component 

 parts. 



While the superior and middle turbinals of Man, that is the 

 four primitive upper olfactory scrolls of Mammals, are to be regarded 

 as neomorphs, the inferior turbinal, below which the lacrymal 

 duct always opens, must be looked upon as derived genetically 

 from that of the lower Vertebrates. It corresponds to the single 

 true turbinal of Urodeles, Reptiles, and Birds, and in Mammals is 

 represented by an independent bone lying at the entrance of the 

 antrum maxillare s. Highmori (Fig. 147, J). 1 



In Man each nasal cavity is divided by the three turbinals 

 into three superimposed passages ; of these the two upper alone 

 (Fig. 147, b, c) conduct the air containing the odoriferous particles 

 to the ethmoidal labyrinth, that is to the proper olfactory 

 region of the nose, while the lower passage serves only as a 

 respiratory tract (Fig. 147, a). 



The nasal chamber of Mammals not only communicates with 

 the maxillary sinus as in the lower Vertebrates, but also with the 

 neighbouring cavities, such as, in Man for instance, the frontal, 

 ethmoidal, and sphenoidal sinuses. These cavities arise in part 

 after birth, and often do not attain their maximum development 

 till the body is fully grown. 2 



1 In Cetacea the turbinals are never more than rudiinentary. 



2 Compare the chapter on air-sacs of Birds, p. 259. 



