In an ambulance on the way to the emergency room, the EMTs are concerned that my 88-year-old mother is having a heart attack. My mother, however, is worried about how her dog, Ajax, is feeling. I tell her that I have arranged for someone to come and pick him up, but she keeps asking, from her gurney in the overcrowded emergency room, if I think he is OK.

Five years ago my sister and I, in a moment of lunacy, bought our mother, the writer Anne Roiphe, a tiny maltipoo in the hopes of assuaging her loneliness in this phase of life. We knew this was a controversial decision. She lived alone. Her mobility was terrible. She could barely walk around her apartment with a cane. How could she take care of a puppy?

My mother, however, was thrilled. Ajax immediately snuggled onto her lap. But he was also an agent of chaos. He chewed her books. He chewed through photo albums. He left half-chewed lamb and turtle toys for her to trip over. He ruined all of her rugs.

But what would these last years have been like for my mother without Ajax?

We generally condescend to animal love. We underestimate it, dismiss it, place it low on the hierarchy of affections, but maybe this is wrong. Ajax occupies an exalted place in my mother’s household. She is far more attuned to his every fluctuation of mood than I am with, say, my children.

Ajax is feted, petted, spoiled to such an extent that he is more maharajah than dog. My mother picks him up so much that he never learned to jump on or off a bed or sofa; he will not walk up stairs. Instead, he waits patiently for someone to carry him. He barks when she eats because she feeds him directly from her plate, though she denies it. He has an unruly mop of white hair just like her. “Ajax is not really a pet,” she once told me.

He gives her day narrative form. The need to feed him, give him water or worry about whether he is sick or tired or bored structures otherwise dreamy and floating patches of time. She ends the day with the warmth of his furry body next to hers in bed. Being needed or doted on by another creature is grounding.

When I invite my mother to a family dinner in Brooklyn, she insists on bringing the dog. To the rest of us, it feels like she is risking life and limb by loading Ajax, along with her walker, into a taxi and possibly getting tangled and tripping on the leash. But she is more worried about him being lonely if she leaves him for a few hours than she is about breaking a hip.

One of the most eloquent meditations on the mysterious potentialities of dog love is Sigrid Nunez’s “The Friend.” An older single woman who prizes the quiet of her one-bedroom life inherits a Great Dane named Apollo . She finds herself taking cabs home instead of the subway because she is so eager to get back to him. She writes, “What are we, Apollo and I, if not two solitudes that protect and border and greet each other?”

In his book “My Dog Tulip,” the writer J.R. Ackerley writes that when his beloved German shepherd died after their 16 years together, he felt like immolating himself on a funeral pyre. No human had ever made him feel that way.

A particular  intensity develops between people who live alone and their dogs . Marilyn Smith, a 78-year-old widow and psychotherapist in Long Island, named her fluffy havanese “Benny,” short for Benzodiazepine, a class of antianxiety drugs. “I know everything he is thinking and he knows everything I am thinking,” she told me. She has three grown boys but jokes that Benny is her “best son.”

My mother’s daughters and grandchildren are busy with work and school and relationships and dinners and trips. Many of her friends are sick or dead. When the human world turns away, when others are busy or distracted or ambivalent, a dog is steadfast . The bond is pure and constant.

In the hospital, my mother tells a nurse about Ajax. “Like the cleaning product?” The nurse asks. “No, the brave warrior from Greek mythology. He is in the Iliad.”

A big name for a tiny fluff ball who can’t jump down from a couch, but he is her protector. He rarely takes his eyes off her. He doesn’t leave her side unless he is out with the dog walker.  He barks at the steam heat coming out of the radiator, but she doesn’t find this annoying. Instead, she feels he is defending her.

As she is being discharged from the hospital, my mother says, “A home without someone you love is not a home but a memory.” She is referring, I think, to the absence of my father, her husband of 40 years, who died in 2005. I don’t know what to say in the face of this painfully accurate assessment. But then, at the thought of Ajax, she brightens , and I am grateful for this dog who is not a dog, this warrior against solitude.