THE NASAL MUCOUS MEMBRANE. 405 



The arterial stems lie in the deeper layers of the tunica propria, from 

 which twigs are sent into the more superficial stratum, where, imme- 

 diately beneath the epithelium, a subepithelial capillary net-work 

 is formed ; other twigs break up into capillaries which surround the 

 glands. The veins are remarkable for their size and number, in 

 many places, particularly over the posterior part of the inferior tur- 

 binated bone, giving to the entire tunica propria the character of 

 cavernous tissue. 



The lymphatics are represented by numerous vessels which con- 

 stitute a net-work within the deeper parts of the tunica propria and 

 around the lymphoid nodules ; in addition to these, within the olfac- 

 tory region wide-meshed net-works of perineurial lymph-chan- 

 nels extend throughout the mucosa of the olfactory region. 

 The olfactory mucous membrane is further provided with a rich sys- 

 tem of intercommunicating lymph- spaces within the groundwork 

 of the tunica propria, which empty into the larger lymphatic net- 

 works of the deeper layers. 



The nerves of the nasal mucous membrane are of two kinds, 

 those providing common sensation and those concerned in the 

 special sense of smell ; the relations of the latter with the neuro- 

 epithelium are of much interest, but at the present time by no means 

 definitely determined. The larger filaments of the olfactory nerve 

 lie against the bony walls, partially embedded within corresponding 

 grooves, and give off smaller arching bundles, which pass within the 

 mucous membrane towards its epithelial surface. The twigs, even 

 within the mucosa in many places, are enclosed by perineurial 

 sheaths prolonged from the intercranial investment of the olfactory 

 nerve. The fibres of the latter are without the medullary sub- 

 stance, being bundles of the axis-cylinder fibrillae enclosed within 

 the neurilemma ; on reaching the epithelium the fibres break up into 

 their component fibrillae, which pass as naked, often varicose, axis- 

 cylinders between the elements of the neuro-epithelium. It is 

 highly probable that the nerve-fibrillae come into close contact, even 

 if not into actual continuity, with the inner ends of the olfactory cells ; 

 whatever the exact actual relation, a very intimate relation between 

 the nerves and percipient elements may be assumed. Additional 

 twigs from the trifacial, composed of medullated fibres, are also 

 distributed to the olfactory region, without, however, coming in 

 relation with the olfactory cells. 



The development of the nasal mucous membranes proceeds 

 from the ectoderm, the earliest indication of these structures ap- 

 pearing as the olfactory plates, two areas of thickened ectoderm 

 immediately above the primitive oral cavity and in contact with the 

 wall of the fore-brain. 



