in the Small Vessels of Animals where he states that the soul, also referred to as the sentient principle, and the body hold equal influence over movement and therefore may govern both voluntary and involuntary action. Voluntary action is movement excited by one's will whereas involuntary action is dependent on the stimulus that is applied to the muscle or nerve of the muscle. He explains that the soul lives concurrently with the body and gives it life. In the brain, the soul has a conscious which is gives us the ability to reason. In the muscles, the soul has the power of producing motion. In the nerves, the soul gives us the ability to feel. Whytt uses the sentient principle to explain the agent that is responsible for movement but he does not address, nor does he feel the need to address, how the soul can act on the physical body. Pupillary light reflex Whytt explains that the pupillary light reflex is the contractions and re-sizing of the pupil in different intensities of light. If the eye was incapable of contraction, we would only be able to see in one degree of light and we would not be able to differentiate the distance of light reflecting from near and far objects. Whytt discovers the pupillary light reflex based on an autopsy of a child suffering from hydrocephalus whose pupils were noted to be unresponsive to changes in light. In the autopsy, Whytt discovered a cyst compressing the optic thalamus in child's eye. Whytt concluded that the obstruction prevented the eye from contracting properly, therefore, limiting the child's eyesight. The pupillary light reflex is later named the Whytt Reflex. Works In 1743 Whytt published a paper in the Edinburgh Medical Essays entitled "On the Virtues of Lime-Water in the Cure of Stone". This paper attracted attention and was published, with additions, separately in 1752, and ran through several editions. It also appeared in French and German. Whytt's treatment of the stone by limewater and soap became obsolete. In 1751 he published a work On the Vital and other Involuntary Motions of Animals. The book attracted the attention of the physiologists of Europe. Whytt dropped the doctrine of Stahl that the rational soul is the cause of involuntary motions in animals, and ascribed such movements to the effect of a stimulus acting on an unconscious sentient principle. He had a vigorous controversy