MELANOSIS, OR MELANOTIC SARCOMA. 437 



In some cases there are no external tumours. The dissection 

 of one case — death occurring from dropsy — revealed a melanotic 

 mass, weighing six pounds, situated in the bronchial lymphatic 

 glands. In another case a tumour was found in the inguinal 

 plexus of glands weighing above eight pounds, which, previous 

 to death, pressed upon the crural nerves, and caused paralysis. 

 In the majority of cases, however, there has been some outward 

 manifestation of the disease ; but in occult disease occurring in 

 grey or very light chestnut horses the practitioner ought to 

 consider whether such disease may not be due to melanotic 

 growths. 



The development of melanosis is looked upon as a new excre- 

 tory function, set up for the purpose of eliminating from the 

 system the pigmentary matter which is no longer required for 

 the purpose of tinging the hair. If it were true that the pig- 

 mentary matter was already formed in the blood, and only 

 required to be appropriated by the structures to which it gave 

 colour, the supposition would be justified ; but the fact is, pig- 

 ment is not conveyed to a part, but is formed by the cells of the 

 tissues in which it is found, out of materials supplied by the 

 blood. We must, therefore, look upon melanosis as' being due 

 to an exalted formative power in the cells of the tissue in which 

 it is found. When formed m the skin of grey horses, we can 

 easily understand that its pigmentary cells may be stimulated 

 to increased activity by the blood, which one may suppose is 

 highly charged with tlie ingredients from which pigment is 

 fovnied. This would be merely an exalted natural action of the 

 cells ; but when tlie pigmentary matter is formed by the cells 

 of glands, or other structures than skin, we must look upon the 

 process as a perverted formative action. 



Melanosis appears usually as a rounded tumour, small at first, 

 but gradually enlarging in every direction within and upon 

 those portions of the body which do not lose their black colour 

 by age : or several of these small tumours may manifest them-: 

 selves simultaneously, enlarging by growth and by coalescence, 

 and thus forming one large tumour. Other tumours form in' 

 the neighbourhood, and iisually extend along the direction of the 

 hairless skin of the tail, anus, and perinteum, imtil at last there 

 is a most unsightly collection of them, like bunches of large 

 onions. After a time they wither in the centre ; the skin. 



