This tiny ancient chapel is known as "Il Capitello" and is inside the Basilica di San Marco, near the left aisle, and houses a miracle-working cruxifix of Byzantine origin.
I found this vintage photo on Flickr where the Architecture Library of Notre Dame has posted thousands of old photos from around the world. I was excited to find it because I'd never seen a photograph of this little chapel before.
Description of Il Capitello from Lorenzetti (Venice and Its Lagoon):
"A pointed marble roof surmounted by a huge oval piece of oriental agate, composed of very rare marbles of great value.
The two columns of African marble with Byzantine capitals support The Virgin and the Announcing Angel, gothic sculptures of the beginning of the XV century.
On the altar table, a painted wooden Crucifix brought into the church in 1290 from Piazza San Marco after it miraculously flowed blood."
Another book says that this crucifix was probably Venice's first and might have been stolen from Constantinople like so many other treasures in the Basilica.
Il Capitello is inside a roped-off section of the Basilica but it's right next to the rope so you can see the crucifix inside as well as the Annunciation sculptures on the front.
Photo from the ND Library: Creative Commons License.