1254 A MANUAL OF ANATOMY 



The veins form a plexus on the outer surface of the tonsil, from 

 which the blood passes into the pharyngeal plexus. 



Lymphatics. — These passto the uppergroupof deepcervicalglands. 



Nerves. — The nerves are derived from (i) the glosso-pharyngeal, 

 (2) the posterior and external palatine branches of Meckel's 

 ganglion, and (3) the sympathetic. 



Structure . — The tonsils are composed of lymphoid follicles. The follicles 

 are ranged upon the sides of the crypts which penetrate into the organ, these 

 crypts being lined with mucous membrane, covered by stratified squamous 

 epithelium. Lymph corpuscles migrate from the follicles into the crypts, and 

 become salivary corpuscles. 



Development. — The tonsil of either side is developed from the epithelium of 

 the corresponding second visceral cleft. About the fourth month a depression, 

 known as the sinus tonsillaris, makes its appearance. Solid epithehal out- 

 growths or buds then extend from this sinus into the surrounding mesoderm. 

 These buds subsequently become hollow; the surface of the sinus becomes 

 pitted ; and so the crypts of the tonsil are formed. The mesoderm in relation 

 to the buds and crypts becomes pervaded with lymphoid cells, and in this 

 manner is formed the lymphoid tissue which constitutes the bulk of the tonsil. 



The supratonsillar fossa is a remnant of the second visceral cleft. 



The Nasal Fossae. 



The nasal fossae extend from the anterior nares to the posterior 

 nares. Anteriorly they open upon the face, and posteriorly they 

 open into the naso-phar5mx. Each fossa is narrow above, but ex- 

 panded below. It presents two walls, outer and inner, a roof, 

 and a floor. The outer wall is rendered very irregular by three 

 convoluted bony eminences, disposed antero-posteriorly, which 

 bulge into the fossa. These are the superior and inferior turbinate 

 processes of the ethmoid, and the inferior turbinate bone. They 

 are also known as the superior, middle, and inferior spongy bones, 

 or conchae. They overhang deep channels, which are known as 

 the meatus — superior, middle, and inferior respectively. 



The superior meatus is confined to the back part of the outer wall, 

 and lies between the superior and middle conchae. It is short and 

 oblique, and opening into it there are (i) the spheno-palatine 

 foramen, which leads from the spheno-maxillary fossa, and (2) the 

 posterior ethmoidal cells, by one or more openings. Above and 

 behind the superior concha is a depression, called the spheno- 

 ethmoidal recess, into which the sphenoidal air-sinus opens. 



The middle meatus is situated between the middle and inferior 

 conchae, is directed from behind forwards, and is overhung by the 

 middle concha. Anteriorly it describes a bend, and passes upwards 

 under cover of the front part of the middle concha, to be continued 

 into the infundihulum, which leads from the frontal air-sinus of the 

 corresponding side. The openings into the middle meatus are as 

 follows: (i) the infundihulum, leading from the frontal air-sinus, 

 with the opening of the anterior ethmoidal cells; (2) the opening of 

 the maxillary air-sinus or antrum of Highmore; and (3) one or two 

 openings of the middle ethmoidal cells. Ihese various openings 



