Fine Structure of Nasal and Oral Cavities in Dog and their Sensory Innervation, especially on the Intraepithelial Fibres

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The mucous membrane of the canine nasal cavity is nearly the samewith that in man in fine structure, except that the former has no smooth muscle fibres as the latter. The nerves in this part are mostly vegetative in nature, while the sensory fibres, very small in number, end in the tunica propria in simple endings, without further entering into the epithelium.
The minute structure of the canine vestibulum nasi is also similar to that in man, except that the former has no pars cutanea with vibrissae, the vestibulum in the front partly going over to the snout, partly to the haired outer skin. The mucosa of the vètibulum is divisible into the corneous part with corneous plate and the non-corneous part, as in the human vestibular mucosa part, but the development of nasal glands is much inferior in the former.
In the submucosa of the vestibulum nasi in dog there are seen many sensory fibres, most of which run toward the corneous mucosa part, to end chiefly with branched endings in the papillae. More intraepithelial fibres are found than in the human counterpart, being in best development at the entrance of the vestibulum. These fibres originate partly in the branched endings in the papillae and partly in single fibres directly penetrating the epithelium. They either end unbranched or branched, are mostly smooth-surfaced and thin, but more rarely somewhat thicker, and run through or between the epithelial cells, to end sharply or sometimes in nodes in the upper layer of the epithelium.
The snout in dog presumably represents the foremost part of the vestibulum. The epithelium is similar to that of the corneous mucosa part of the vestibulum, being composed of stratified flat epithelium with corneous plate and somewhat greater in height than the latter. The development of papillae is much stronger than in the vestibular corneous part and their arrangement is very regular. No nasal gland is found formed in the tunica propria of the snout.
The snout is far better provided with sensory fibres than the vestibular mucosa part, apparently in proportion to the good development of papillae here. The sensory fibres mostly end in the papillae as branched endings, but in rarer cases, as capsulated end bulbs. Often enough, these endings send out intraepthelial fibres, very large in number, most of which are branched. Some of them are rather thick, though mostly they are minute in size, and generally end sharply or in nodes in the upper layer of the epithelium.
The canine lip, as that in man, is composed of the cutanea part, the transitional part and the mucosa part. The transitional part is subdivided into the outer and inner zones by a shallow furrow. The outer zone occupies only a narrow space. Its epithelium is composed of a stratified flat epithelium with corneous plate and about 3 times thick as the epidermis of the cutanea part. Rather strongly developed papillae are formed beneath the epithelium. Upon going over from the outer to the inner zone, the epithelium suddenly grows in thickness, reaching 4 to 5 times that of the epithelium in the outer zone. No corneous plate is found herein, and villi are formed on the surface. The papillae here are tall and well-developed. The inner zone occupies a far larger area than the outer. Its epithelium diminishes in thickness as the mucosa part is approached, and its height becomes nearly equal to that of the outer zone upon going over to the mucosa part. In the mucosa part, no papillae are formed, unlike that in human lips. Of the mucous membrane of the cheek outside the lip mucosa, only the part corresponding to the level above the rima oris contains papillae.
The sensory supply of the mucous membrane of the lips and cheeks in dog is similar to that in man, the inner zone of the transitional part, the outer zone thereof and the mucosa part being best innervated, in the order named.