Now we get to the technical reason that made Bouguereau paintings so realistic. More than any other painter Bouguereau showed veins and arteries, which we know by our study of anatomy, lie just below the skin and show behind the knee, in the neck, inside the elbow, in the lower abdomen and between the thigh and the torso. There was no need to show them all, yet just sufficient to give the model a defined 'humanity'.
This is a large step away from the 'ideal' toward the 'real' and needs to be handled with the same care a jazz musician might use a discord to amplify a harmony. Not too much that might make it look like a blue road map.
No doubt the display of veins and arteries is not one that would normally attract the viewer BUT in this case it has another purpose. It throws into the mix of the ideal figures and perfect forms something that might otherwise be thought ugly. In Fig.7 you can see where I have shown the veins that occur between the thigh and torso of the female, and the thigh of the male.
Also note in Fig.8 and elsewhere I have reddened hands, feet, knees, elbows and faces as the blood is closer to the surface in these more sensitive areas.
How is this done with paint?