This is a remarkable essay - way ahead of its time. The author deduces that Parkinson's disease is a distinctive condition with clearly defined symptoms and, presciently, that it is intrinsic to the brain (not caused by, for example, excessive alcohol or tea/coffee consumption), although his thoughts on treatment and possible cure are a stab in the dark - hardly a surprise as, at the time, there was almost no understanding of brain function. He also notes the characteristic symptoms, such as the distinctive gait of sufferers and the counter-intuitive fact that supporting a shaking limb actually increases the tremor, and their progression over time.
I have two relatives with the disease and, I must say, I was stunned by the readability and accuracy of the clinical description.
90 years after publication it is remarkable how well the composers chosen have stood the test of time with a couple of exceptions - Ornstein and Loeffler have largely sunk without trace. The author's personal prejudices and, strangely, anti-Semitism are sometimes overt (witness the gratuitous attack on Reger as "... a swollen myopic beetle with thick lips and sullen expression crouching on an organ-bench" for one!) and the detailed opinions have frequently dated or seem superficial in retrospect (the old notion that Mahler imperfectly assembled an incoherent mass of bits and pieces in his symphonies, for one) but, overall, this book is a fascinating period piece.