no BIOLOGY OF THE LABORATORY MOUSE 



ill section as homogeneous brown drops. The short excretory ducts are 

 lined by cuboidal epitheHal cells and open at the base of the nictitating 

 membrane. 



Oral Cavity and Associated Structures 



The oral cavity. — The lips are covered on the outside b}' skin containing 

 deeply imbedded hair follicles. At the zone of transition from the skin to 

 the mucous membrane covering the inner surface of the lips, the hairs dis- 

 appear and the stratified squamous epithelium becomes much thicker, its 

 outer layers being cornified. Similar epithelium lines the entire oral 

 cavity. Below the epithelium the fibrous lamina propria forms low, 

 broad papillae. At the corners of the mouth there are large sebaceous 

 glands which open directly through short ducts to the surface of the lips. 



The dental formula of the mouse is: incisor i/i, cuspid o/o, premolar 

 o/o, molar ^/_^. The incisors in both jaws are bow-shaped with the root 

 projecting far back below the root of the third molars. According to Weber 

 (in Jaffe, 56) the crown on the outer convex side is covered by enamel, while 

 on the inner concave side the enamel is absent and the dentine is covered by 

 cementum. As the incisors are growing continuously, their apical foramina 

 stay open. The molars are similar structurally to the human molars. 



The anterior part of the roof of the mouth, the hard palate, bears rows of 

 membranous ridges. The three anterior ridges are transverse; the five 

 pairs of posterior ridges are V-shaped. They are covered by stratified 

 squamous epithelium showing keratinization, and are supported by the 

 dense fibrous lamina propria which takes part in their formation. The 

 mucous membrane is firmly attached to the surface of the bones. 



The posterior part of the roof of the mouth, the soft palate, is composed 

 of striated muscle fibers and fibrous connective tissue covered by mucous 

 membrane. On the oral surface and at the posterior margin of the soft 

 palate the epithelium is cornified stratified squamous, while toward the nasal 

 surface, a short distance from the margin, this changes into columnar 

 pseudostratified, and still farther into pseudostratified ciliated columnar 

 respiratory epithelium. Mucous glands are present on the oral surface 

 beneath the mucous membrane. They are surrounded by loose vascular 

 connective tissue and open through short ducts into the oral cavity. 



The tongue. — About one third of the distal part of the tongue lies free 

 in the oral cavity. Farther caudad it is attached to the floor of the mouth 

 and the wide proximal part is attached also on the sides, here forming the 

 floor of the mouth cavity. The tongue is covered by stratified squamous 



