What type or class of chemical messenger traveling in the blood would most probably link the brain with the digestive tract and fat cells in the control of body weight?

Hormones.

Explanation:

Hormones may be defined as the chemical messengers of the body that causes the action of the specific tissue. The hormones can be directly transfer into the blood or may be excreted into a duct.

These hormones can travel in the blood that can link the brain with the digestive system and fat cells that control the weight of the body. The abnormal functioning of these hormones can cause a particular disease.

Thus, the answer is hormones.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.