Emerson got inside the trailer and began checking pulses and breathing of the "motionless" bodies on the floor.
"Due to how packed together the bodies were in the trailer, it was not possible to check everybody, so I made an attempt to check the bodies I could reach," he added.
Most of the victims were lying on their backs, some appeared to be frothing from the mouth, and some still warm, he told jurors.
Truck driver Maurice Robinson, 26, was arrested at the scene and has pleaded guilty to 39 counts of manslaughter.
He had collected the sealed container from the southeast English port of Purfleet after it arrived on a cargo ship from Zeebrugge in Belgium.
Post-mortem examinations found the migrants, who included two boys aged 15, had suffocated to death in unbearably high temperatures during the journey.
Prosecutors said Robinson waited 23 minutes before alerting police, after making the grim discovery on an industrial estate east of London.
Haulage firm boss Ronan Hughes, 41, has also admitted 39 counts of manslaughter.
But Eammon Harrison, 23, who is said to have driven the truck to Zeebrugge, and Georghe Nica, 43, both deny the same charges.
Harrison, Valentin Calota, 37, and Christopher Kennedy, 24, deny being part of a people-smuggling conspiracy, but Nica has admitted that charge.