DISTRIBUTION OF THE SCHNEIDERIAN MEMBRANE. 455 



nan membrane within. Here the spheno-palatine nerves and 

 arteries join the membrane. Below this meatus, it extends 

 over the middle, (formerly called the upper,) turbinated bope, 

 and is reflected or folded inwards on the under side of this bone, 

 and continued into the middle meatus below it. In the middle 

 meatus, which is partly covered by the last mentioned turbina- 

 ted bone, there are two foramina ; one communicating with the 

 maxillary sinus, and the other with the anterior cells of the 

 ethmoid and the frontal sinuses. The aperture into the maxil- 

 lary sinuses is much less in the recent head, in which the 

 Schneiderian membrane lines the nose, than it is in the bare 

 bones. A portion of the aperture in the bones is closed by the 

 Schneiderian membrane, which is extended over it : the re- 

 mainder of the aperture is unclosed ; and through this foramen, 

 the membrane is reflected so as to line the whole cavity. As a 

 portion of the foramen is covered by the 'membrane, and this 

 portion, as well as the other parts of the cavity, is lined by the 

 membrane, it is obvious that at the place where the membrane 

 is extended over the foramen in the bone, it must be doubled ; 

 or, in other words, a part of the aperture of the maxillary sinus 

 is closed by a fold of the Schneiderian membrane.* 



This aperture varies in size in different subjects, and is often 

 equal in diameter to a common quill. It is situated in the 



gland, placed under the tongue, and communicating with the mouth by a small 

 duct, (ductus Bartholinus :) many small ducts from this gland, open into the 

 duct of the gland below, m, Sub- maxillary gland, situated below and behind 

 the preceding gland, n, Thyroid gland, s, Vertical section of the border of 

 the cervical vertebrae, to which the pharynx is attached by cellular tissue. 

 tj Spinal canal, u, Section of spinous processes and muscles of the neck. 

 v, Left nostril. TV, Bony palate, z, Trumpet-shaped orifice of the Eustachian 

 tube, y, Inferior turbinated bone, covered by the Schneiderian membrane. 

 z, Middle turbinated bone. 1, Superior turbinated bone, both covered with the 

 same membrane. 2, Superior meatus. 3, Middle meatus. 4, Inferior meatus. 

 5, Place of opening of the ductus ad nasum. 6, Frontal sinuses. 7, The 

 posterior nares. 8, Sphenoidal cell in the body of the sphenoid bone, showing 

 the orifice below, by which it communicates with the top of the pharynx ; above 

 is seen the sella turcica of the sphenoid bone. 



* In the mucous membrane lining the cavities of the maxillary, sphenoid 

 and ethmoid bones, no one has yet detected any mucous follicles. The pouch 

 formed by the reflection of the membrane, seems itself to constitute a large 

 follicle, from which mucus is abundantly secreted. r. 



