76 CLINICAL APPLIED ANATOMY. 



danger of removing them unless there is absolute asepsis. The 

 other fissures of the face much more rarely have true dermoids 

 associated with them. 



The branchial clefts in the neck may sometimes develop der- 

 moids lined wholly with skin, but more frequently the cysts formed 

 in connection with them are of the nature of tubulo-dermoids, 

 sometimes called " hydroceles " of the neck. 



Dermoids may be occasionally seen about the pinna of the ear, 

 and occur there owing to sequestration of epithelium during the 

 fusion of the primitive auricular tubercles. 



Dermoids in connection with the scalp and dura mater may be 

 explained by the original continuity of these two layers, which 

 later become separated by the formation of membrane bone 

 between them. 



Implantation cysts, which bear a resemblance to sequestration 

 dermoids, are found in various parts of the body, and are due to 

 portions of epithelium being driven by violence into the deeper 

 tissues, or left behind accidentally during the progress of an 

 operation. Hence such cysts are usually seen about the palm of 

 the hand and the fingers, because these parts are so exposed to 

 cuts and pricks. 



Tubulo-dermoids. These arise in congenital tubes, which 

 as a rule become obliterated before birth. There are chiefly 

 three which may be the origin of tubulo-dermoids, the thyro- 

 glossal duct or tract, the post-oral fissures, and the post- 

 anal gut. 



From the first may arise tubulo-dermoids in the middle line of 

 the base of the tongue, in front of the thyro-hyoid membrane, 

 and in the middle line of the neck below this and down to the 

 isthmus of the thyroid body. It is important to recollect that 

 the cyst may pass up behind the hyoid bone into the substance 

 of the tongue, and to rid the patient of the trouble necessitates 

 complete extirpation of the dilated duct, and not a mere opening 

 of the fluid swelling, which is so commonly mistaken for an 

 abscess. 



Similar cysts have been described with pedicles passing between 



