270 



COMPARATIVE ANATOMY 



but with the reduction of this sense, they lose tleir primary 

 function, often persisting merely as air-sinuses or wen disap- 

 pearing entirely l (Pinnipedia). 



The nasal glands of Mammals may be divided into two sets, 



numerous small, diffuse Bowman s glands, and a large Stcnson's 



aland. The latter appears early in the embryo, and in many cases 



undergoes reduction ; it is 

 situated in the lateral or 

 basal walls of the nasal 

 cavity, opening into the 

 vestibule of the nose, and 

 may extend into the maxil- 

 lary sinus. 



The presence of an ex- 

 ternal nose (cf. p. 131), which 

 must be regarded as a de- 

 rivative of the outer nasal 

 chamber of Reptiles and 

 Birds, is very characteristic 

 of the olfactory organ of 

 certain Mammals, that of 

 FIG. 199. TRANSVERSE VERTICAL SECTION Man being of a specialised 

 THROUGH THE NASAL CAVITY OF MAN. type not exactly comparable 



a, I, c, inferior, middle, and superior nasal to tne SO-Called external 



passage ; C.cr, cranial cavity ; HG, hard nose of other Mammals, 



palate ; /, //, ///, inferior (maxillary), ft j g supported by an out- 

 middle, and superior turbmal ; J, J, , . f j , , 



position of vestigial Jacobson's organs, ward extension 01 the nasal 



which are situated nearer the floor of the bones and by the cartila- 



cavity than is indicated in the figure; M, gi nous septum nasi which 



maxilla; Or, wall of orbit; S, septum & ff ,, ., , 



nasi ; SL, ethmoidal labyrinth ; *, point anses n ' om the ethmoid, by 



at which the nasolacrymal duct opens ; the roofing lateral nasal 



t, entrance into the maxillary sinus (C.m). cartilages connected with 



the septum, and by the 



vomer, as well as by other secondarily independent cartilages 

 (alinasals), which were primarily continuous with the general 

 solid cartilaginous wall, but become differentiated from the latter 

 in various ways in accordance with varied functional adaptations. 

 The outer nose contains a paired cavity (vestibule) continuous with 

 that of the olfactory chambers, and may be provided with a 

 complicated musculature, which in diving Mammals forms a 

 sphincter in connection with a special valvular apparatus for 

 closing the nostrils. An excessive development and increase in 

 the musculature, as well as an upward and backward shifting of 

 the nasal apertures in their relation to the skull, is seen in those 



The maxillary sinus is the most constant, and is typical for the Eutheria : 

 it usually extends into a number of neighbouring bones. In general, the pneu- 

 maticity of the skull is in direct proportion to the size of the animal : in Insect i- 

 vores and Bats the maxillary sinus is the only one present. 



