78 CLINICAL APPLIED ANATOMY. 



CYSTS. 



Cysts may be conveniently classified under three headings, 

 firstly, cysts due to distension of previously existing cavities ; 

 secondly, cysts of new formation ; and thirdly, cysts of congenital 

 origin. 



Retention Cysts occur wherever there is a secreting organ 

 which possesses a duct, the lumen of which becomes occluded, 

 while at the same time secretion continues. Possibly the most 

 frequent form of this variety is seen in the common sebaceous 

 cyst of the skin, or the mucocele of the mucous membrane. 



Distension Cysts are those in which a potential cavity, lined 

 with a secreting membrane but with no exit therefrom, becomes 

 distended with its own secretion. A typical example of this is 

 distension of the tunica vaginalis testis with serous fluid, 

 forming a vaginal hydrocele. 



Cysts of New Formation are those originating in connection 

 with a new growth or a parasite, an excellent example of the 

 latter being a hydatid cyst. 



Cysts of Congenital Origin are very numerous, and for the 

 most part depend upon the dilatation of developmental tubes 

 which should cease to exist or remain merely as remnants. 

 Congenital neural cysts are due to arrest of the fusion of the 

 edges of the neural groove. Spina bifida usually occurs in the 

 lumbar region where the neural plates are the last to close. The 

 plates are also late in closing at the anterior end of the groove, 

 and a meningocele or encephalocele may form at this site and 

 project at the root of the nose or into the nasal cavity. A 

 cerebral meningocele may also project between the two cartila- 

 ginous portions of the supra-occipital bone. 



