The common spindle-cell tumors of the uterine muscular wall.
The first picture shows a uterus, bisected and opened for examination. The cervix is at the bottom. You can see three round white leiomyomas.
Under the microscope, you can see that a leiomyoma is made of interlacing bundles of smooth muscle cells. This imparts the distinctive whorly pattern seen grossly. Some people compare this to watered silk.