486 HISTOLOGY 



The deeper layers of the tunica propria contain the arteries of the 

 mucous membrane, which send branches toward the epithelium, and 

 form a thick sub-epithelial plexus of capillaries. The veins are very 

 numerous, especially at the inner end of the inferior concha, where the 

 tunica propria resembles cavernous tissue. Lymphatic vessels form a 

 coarse meshed network in the deeper connective tissue. Injections of the 

 arachnoid spaces around the olfactory bulbs follow the perineural sheaths 

 of the olfactory nerves into the nasal mucosa, but these tissue spaces are 

 not lymphatic vessels. 



The olfactory nerves, as already stated, are formed of the basal proc- 

 esses of the olfactory epithelial cells, which become non-medullated nerve 

 fibers. This is a primitive type of nervous apparatus (cf. p. 132), such as 

 is not found elsewhere in the human body. After a tangential course 

 beneath the epithelium, the fibers unite in bundles, and pass through the 

 cribriform plate of the ethmoid bone to the olfactory bulb just above it, 

 which they enter. They spread tangentially and branch, finally termi- 

 nating in the glomeruli. The glomeruli are round or oval groups of 

 arborizing fibers, in which the processes of the olfactory cells end in 

 relation with the dendrites of the mitral cells. The latter are nerve cells 

 with triangular bodies, which form a characteristic layer of the olfactory 

 bulb, and send their neuraxons through the olfactory tracts to make 

 various connections within the hemispheres. 



In addition to the olfactory nerves, the nasal mucous membrane con- 

 tains medullated branches of the trigeminal nerve, distributed both to the 

 olfactory and respiratory regions. 



