According to the textbook:

• Pons (A) consists of fibre tracts that interconnect different regions of the brain (III) .
• Hypothalamus (B) contains several groups of neurosecretory cells (IV) which secrete hormones .
• Medulla (C) (oblongata) contains centres which control respiration, cardiovascular reflexes, and gastric secretions (II) .
• Cerebellum (D) has a very convoluted surface to provide the additional space for many more neurons (I) and is involved in balance .