Reuben Crosby's cause of death was "gored by a bull." This is listed on the death record of my 5th great-grandfather in 1867. This cause of death was quite easy for me to understand.
However, on most of the death records I've viewed, the cause of death ranges from billiousness, to diptheria, to poliomyelitis -- certainly terms that are not as common today, and even have modern equivalents.
A good resource for understanding the medical terms used on your ancestor's record of death is found at Old Disease Names and their Modern Definitions.
Other links to diseases and medical terms is found at http://www.cyndislist.com/medical.htm#Diseases.