Give scientific reasons:
$(1)$ Hormones are regulatory chemicals.
$(2)$ Less secretion of thyroxine results in stunted growth of a person.
$(3)$ Deficiency of insulin results in diabetes mellitus.
$(4)$ Pancreas is an exocrine and endocrine gland.

(D) $(1)$ Hormones regulate various physiological processes through two methods. Some physiological activities are performed by the release of stimulating chemicals,while some inhibitory chemicals regulate the rate,amount,and type of activities. Thus,they regulate various physiological activities by stimulating or inhibitory actions,hence they are called regulatory chemicals.
$(2)$ Deficiency of thyroxine hormone results in rough,abnormal skin,a thick tongue,and faulty respiration. If such deficiency is observed during development,it leads to cretinism (dwarfism and mental retardation). In adults,it causes myxedema,characterized by an abnormal face and inflammation of body parts. Thus,reduced secretion of thyroxine significantly affects the growth and metabolism of an individual.
$(3)$ Lack of insulin or reduced secretion leads to an increase in blood glucose levels (hyperglycemia). Excess sugar is excreted in the urine (glycosuria). Symptoms include increased thirst (polydipsia),increased hunger (polyphagia),and frequent urination (polyuria). Long-term effects include damage to the heart,kidneys,and eyes. These are the characteristic symptoms of diabetes mellitus.
$(4)$ Pancreatic acini found in the pancreas secrete pancreatic juice containing digestive enzymes,which is transported via the pancreatic duct to the duodenum. Thus,due to the presence of a duct,it functions as an exocrine gland. The Islets of Langerhans secrete hormones like insulin and glucagon,which are released directly into the blood to reach target organs. Thus,it also acts as an endocrine gland.