The cataract, now removed, is milky and kernel-shaped--as if the eye, like an oyster, had transformed sand into a pearl--but that is the smallest of the mysteries in Dr. Koch's veterinary surgery.

Here a springer spaniel, eyes crusted and angry because of tear gland failure, is given new irrigation by the transposition of the salivary ducts. Here a retriever's ruined eye is replaced by an aggie-sized prosthesis, which is popped in through the wide-stretched circular muscle like a birth in reverse.

Dr. Seth A. Koch, 45, is a veterinarian whose practice is limited to ophthalmology--dogs, cats and the occasional horse. One of only a handful of animal ophthalmologists in the eastern United States, travels like a modern-day circuit preacher--Owings Mills to Baltimore to Gaithersburg to Fairfax to Charlottesville to Richmond to Crofton, Md.--even holding regular clinic hours in New York City every other Monday.

His permanent floating Animal Eye Clinic touches down Thursdays (examinations) and Fridays (surgery) at Dr. David E. Jackson's veterinary clinic at Fairfax's University Mall.

On this particular Thursday, with a mere 20 appointments, he predicts it will be "a dead day"--but one breeder brings in 21 huskies, another a half-dozen collie pups; a Rockville veterinary clinic sends over an emergency, a perforated cornea; and Koch squeezes in unscheduled bilateral surgery on a peekapoo whose owners have driven up from near Portsmouth.

In between, Koch reports to other veterinarian who have requested special exams or second opinions and suggests to a caller in upper Maryland that he attempt to treat his dog's irritation with "a drop of Mazola in each eye."

"This," says Koch cheerfully, "is the only profession in the world that gives free advice over the phone."

Koch may have, as he admits with perverse pride, "no bedside manner," but his Jekyll and Hyde style--frank and painstaking with humans, Mel Brooks with animals--seems reassuring to both.

"The bottom line in veterinary medicine, unfortunately, is economics," he says. "Especially as the pet grows older, people have to think about how much they want to put into an 'older patient,' so to speak."

To each owner, Koch delivers a complete explanation of his pet's condition, all options, medical or surgical, the price of every procedure and the risks involved. He writes everything down, answers all questions.

The owners of the peekapoo, Lil Bit, for example, are told their 12-year pet dog has cataracts in both eyes and, in one, glaucoma and a slipped lens. Corrective surgery on both eyes, which will restore some vision in one, plus the hospitalization, anesthesia and the preliminary EKG (transmitted over the telephone to a cardiologist in New York), will cost about $600.

"We've already spent $75" on blood and kidney tests, says the man, a husky farmer in a plaid shirt and a Donald Duck pin on his suspenders. Six hundred dollars is "more than my wife and I have spent on ourselves in three months."

But they don't hesitate. "I've put a hell of a lot more money into things that I never got any pleasure out of," the man says, and they drive away, leaving Lil Bit for the night.

"You know, the princesses of the world walk in here with their Guccis on, and when they hear an operation may cost $120, they gasp," says Koch, not bitterly but almost wearily. "To them, the dog is just another accoutrement. To Lil Bit's owners , it's their child."

Koch is, as he says, the kind of man you either love or hate. Balding with salt and pepper beard, he looks like Pernell Roberts and talks like Carl Reiner.

"All animals are 'she,' don't ask me why," he says after an unwary owner corrects him. "Oh, what a good dog," he croons to a patient Kerry terrier. "Hold it, wiggly-tush," he scolds a frantic puppy. "Oh, God, what an ox!" he groans, lugging an anesthetized Labrador to the steel surgical table. (Back trouble, according to Koch, is the occupational disease of the veterinarian: he's already had surgery twice.)

Although Koch, who began his ophthalmic practice in 1969, was the first veterinary specialist in the region, he is now one of perhaps a dozen--neurologists, cardiologists, internists, etc.

The great paradox of veterinary medicine, according to Koch, is that general practitioners--the old-style neighborhood veterinarians--make far more money than the specialists. Koch estimates that he grosses $120,000 a year, but nets only about $40,000.

"That's nothing compared to the GP--it's just the opposite of the medical professions," Koch says. "And the reason is I don't do immunizations, I don't do de-claws, I don't do spays."

This more general practice is the domain of Koch's wife, Dr. Linda de Chambeau, who runs the 800-square foot Family Animal Clinic at their home in Crofton, Md.

For all the schtick, Koch has a still center: Like a hurricane, he harnesses that restless energy to the meticulous demands of his surgery. His work is full of a startling, offhand beauty--in a darkened room, the sudden flame of a retinal reflection.

"One thing about Dr. Koch," says a staffer at the University Clinic. "Behind all that mouth--he's good."