Question on a 19th century medical joke

In a book published in 1854 is the following item:

http://ift.tt/2xe2jY5

Quote:

A New Syrup. — Very much confounded was Dr. Doane, a few years since, by a remark of one of his patients. The day previous, the doctor had prescribed that safe and palatable remedy, the syrup of buckthorn, and left his prescription duly written in the usual cabalistic characters — " Syr. Rham. Cath." On inquiring if the patient had taken the medicine, a thunder cloud darkened her face, i lightning darted from her eye, as she roared out : " No ! I can read ! your doctor writing, and ain't a going to take the Syrup of Ram Cats for any body under heaven !"

My question is this; what does the medical direction, " Syr. Rham. Cath." actually mean or direct someone to do?

Syr. appears to mean syrup but what is the meaning of Rham. Cath.?...catheter??

Thanks

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