378 



Tumors. 



presumed that irregularities of their loss of pigment underlies 

 the anomaly. Melanoma may, however, also occur in brown 

 horses and has been seen once in a black (Hall). In most 

 instances these growths develop in horses in the region of the root 

 of the tail, the anus and external genitalia. In these positions 

 they form nodes which protrude like boils beneath the skin, 

 attaining the size of several fists and several kilograms in weight. 

 Secondary nodes are formed along the lymph passages, so that 

 in the cellular tissue of the pelvis there may be found whole 

 chains of tumor nodes ; and by infiltrative growth the tumor 



Fig. 109. 

 Section through horse's tall involved by melanoma. 



may completely invade the cutis and subcutaneous tissue, and 

 cause widespread thickening and swelling. The growth is 

 usually slow ; it may continue for several years as a circum- 

 scribed and strictly local process, at other times being highly 

 metastatic and causing an extensive generalized melanosarco- 

 matosis, with secondary nodes in the lung, pleura, spleen, liver, 

 kidneys, bones, musculature and elsewhere. Melanomata of the 

 same nature as the above have been met also as primary growths 

 in the cellular tissue near the parotid gland, in the pancreas 

 (Kasewurm), intestine (Csokor), at the base of the heart (Dex- 

 ler), and in the spleen. 



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