ORGAN OF SMELL IN MAMMALIA. 205 



here they divide, subside, expand, and anastomose with each 

 other, forming a reticulate nervous expanse, with long and narrow 

 meshes, and becoming impacted in the central or inner layer of 

 the olfactive membrane. This membrane is continued into the pi- 

 tuitary one, covering the inferior spongy bone or 'maxillo-turbinal' 

 supplied mainly by the fifth. Both tracts, and especially the 

 latter, are richly supplied with arteries opening into numerous 

 large plexiform veins on the peripheral side of the membrane, 

 occasioning or resembling, there, a cavernous structure, and 



O o- 7 



admitting of such change in the quantity of blood therein as must 

 be attended with concomitant degrees of laxity or tension of the 

 scentino- membrane itself. 1 This at the attachment of the tur- 



o 



binals is continuous with the lining of the nasal chamber ; which 

 itself becomes modified into the more delicate and still less vas- 

 cular membrane of the contiguous or accessory air-sinuses. The 

 nasal membranes are finally continued at the posterior aperture 

 into the mucous membrane of the fauces and pharynx, and at the 

 anterior one into the integuments of the face. The pigmental 

 layer of the skin is soon lost within the nose, the colour of the 

 pituitary and olfactory membranes being due -to the abundant 

 blood sent to them. Numerous mucous crypts are imbedded in 

 the pituitary part of the nasal membrane. 



The cavity containing the organ of smell is formed by the 

 prefrontal, vomerine, nasal, sphenoid, pterygoid, palatine, max- 

 illary, and premaxillary bones, and may be continued by exten- 

 sion of air-sinuses into all the bones of the cranium, figs. 1 54 and 

 157. The cavity is divided by a medial partition of bone and 

 gristle in varying proportions, the bone being contributed by the 

 prefrontals, the vomer, and by ridges of the nasals, palatines, 

 maxillaries, and premaxillaries, with which the vomer may 

 articulate. Each half of the cavity is a passage for the respiratory 

 currents of air, opening anteriorly upon a more or less produced 

 and mobile part called ' nose,' ' snout,' or f proboscis,' and pos- 

 teriorly into a cavity containing the larynx or beginning of the 

 windpipe; sometimes, as in Cetacea and in Marsupials at their 

 mammary stage, containing the larynx exclusively, but commonly 

 communicating also with more or less of the pharynx. In the 

 section of the human skull, fig. 152, the outer wall of the right 

 nasal passage is shown, with the communicating frontal, 3, and 

 sphenoidal, 4, sinuses ; i is the nasal bone, and a the nasal spine 



1 Lxxxii--. p 278, and LIV. p. 123. (The second edition of this valuable aud 

 original work, 4to, 1864, is the one cited in the present volume.) 



