Why is the endocrine system considered a chemical extension of the nervous system ?
The functions of the body are controlled and coordinated by nervous system. However, the chemical control is done by the hormones secreted by endocrine glands. The hormones are chemical messengers and regulators.
For example the hypothalamus, which is a part of nervous system secretes TSH-RH (Thyroid stimulating hormone - releasing hormone). The hormone is transported to pituitary gland where it stimulates anterior pituitary to secrete Thyroid stimulating hormone TSH. The TSH in turn stimulates the thyroid gland to secrete thyroxin. Thus, endocrine system and nervous system operate in coordination and many functions of the endocrine system are under control of nervous system. The two are collectively called as neuro-endocrine system. Thus, endocrine system is a chemical extension of nervous system.