Sentence:  Patients of this description derive a marvellous amount of benefit from hydrotherapy, and, when it can be had, from maritime hydrotherapy : this treatment <WSD>tonifies</WSD> their whole system, and enables them to resist variations of temperature, without contracting the intestinal catarrh from which, under similar circumstances, they were previously in the habit of suffering. 
Meaning of the word marked with <WSD> tags:  (more generally) To invigorate; to have a restorative effect on.