ZOOLOGICAL DISTRIBUTION OF DISEASE. 267 



whilst in other parts they are absent. Not infrequently 

 changes occur in the interior of such glands causing the 

 sebum to be retained, and the acini of the glands 



FIG. 130. A Mouse with a cutaneous horn arising in a seba- 

 ceous cyst. 



becoming dilated by the accumulated secretion forms 

 a perceptible swelling known as a sebaceous cyst. At 

 times the secretion bursts through 

 the cyst-wall in the form of a 

 dark-coloured corneous substance, 

 which increasing in length, by ad- 

 ditions at the base, may attain a 

 length of many centimetres. Such 

 a horn is shown in fig. 130, grow- 

 ing from a cyst on the back of a mouse, and in order to 

 show its relation to the sebaceous cyst, the horn is shown 

 in vertical section in fig. 131. 



Cutaneous horns, in their naked eye and even micro- 



FIG. 131. The horn and 

 cyst in section. 



