SENSITIVE SYSTEM. 363 



NOSTRILS. — Four in number: — two on each side, distin- 

 guished by the epithets true mid false. 



The true nostrils are the large, ovoid, and ever-open 

 orifices so conspicuous externally. They have for the base of 

 their structure four pieces of fibro-cartilage, which are involved 

 in doublings of the common integument. Each nostril is formed 

 of two flexible ala or wings : — a superior or internal one ; and 

 an inferior or external : the former is supported by a broad cir- 

 cular cartilaginous plate ; the latter is crescentic in shape, and 

 forms a flexure outward, within which is perceptible the orifice of 

 the lachrymal duct. They are attached to, and supported by, 

 the nasal peak and septum nasi. 



The false nostrils are two little pouches or cavities (having 

 the semblance of cuk-de-sacs J, situated internally, above the true 

 nostiils, into which an external opening is found within the 

 commissure formed by the union of the two alse. They are 

 formed out of duplicatures of the skin, which is here thinner, 

 and finer, and softer in its texture ; and, except at their entrance, 

 are without hair upon their surfaces. Their use is not known. 



SCHNEIDERIAN MEMBRANE.— The cavity of the nose 

 is not only divided into the two nasal fossse, but each fossa is 

 subdivided into the three meatus (for a description of which, 

 vide page 45). Every part of these cavities and passages is 

 covered by the Sclmeiderian or pituitary membrane. This is a 

 membrane of the mucous class, distinguished for its thickness 

 of substance, for its vascularity, and for its olfactory papillae. It 

 has two surfaces : — an exposed or secreting one ; and an un- 

 exposed or adherent one. The secreting surface is smooth ; and 

 is rendered glib and shiny by the varnish it derives from the 

 mucous secretion emitted by the numerous small rounded pores 

 everywhere visible in the membrane, but more particularly upon 

 the lower part of the septum, and upon the inferior turbinated 

 bone. This surface exhibits a pale pink blush, the effect of the 

 bloodvessels spread over it, which are here so superficial as to 

 owe their principal defence to the mucous exudation : hence it is 

 that the complexion of the membrane (varying with the influence 

 of the atmosphere and other agents) is extremely fugitive and 

 uncertain. The adherent surface of the membrane contracts a 

 close and firm adherence to the parts it covers, through the insi- 

 nuation of its fibres into them : indeed, to the bone it appears to 

 supply the place of periosteum; to the cartilage, of perichon- 

 drium. The substance of the membrane exhibits a fibrous struc- 

 ture, interwoven with cellular tissue ; and upon that — as a sub- 

 stratum — is spread a glandular and vascular apparatus, from 

 which issues the mucous secretion ; together with numerous pa- 



