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 
CHRONO eee 
— os - MORAN 
Fic. 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 Fic, 131.—The horn and 
a horn is shown in fig. 130, grow- eyst in section: 
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- 
