Iodine is needed by the thyroid gland for the production thyroxin hormone. This hormone regulates fat, protein and carbohydrate metabolism in our body. If this is in deficient quantity then the neck will swell up due to the enlargement of thyroid gland.

Central nervous system- central nervous system consists of the brains and spinal cord. brain is the main controlling part of nervous system and it is safely located in the cranial cavity. In the space between the delicate central nervous system and its bony covering are the protective layers called the meninges.cavities present in various parts of the brain are called as ventricles. the long tubular cavity of spinal cord is called as central canal. spinal cord is held within vertebral column. it is slightly thick but gradually tapers towards the end. there is thread like fibrous structure at its end called as filum terminal.

peripheral nervous system- the peripheral nervous system consists of the nerves originating from the central nervous system. these nerves connect the central nervous system with all parts of the body. they are of two types-A) cranial nervesnerves originating from the brain are called cranial nerves.bthey are associated with various parts in the head , thorax and abdomen. these are 12 pairs of cranial nerves.B) spinal nervesnerves originating from the spinal cord are called spinal nerves. these are associated with the arms, legs, stomach, etc . it is not under the control of our will.