In open circulatory system, the blood (i.e. hemolymph) bathes the organs and tissues directly. Examples of animals with open circulatory system are arthropods and most mollusks. In closed circulatory system, the blood circulates within blood vessels throughout the body. Therefore, there is a distinction between blood and lymph. Examples of animals with closed circulatory system are annelids, few mollusks, and vertebrates.
Its main task is the protection of the most important organ in the human body: the brain. The brain is almost entirely enclosed by the cerebral cranium with the exception of the foramen magnum and other foramina at the skull base which serve as entry and exit point for blood vessels and cranial nerves. The braincase consists of the skullcap (calvarium) and the skull base.
The skull can be divided into two main parts: the cranium and the mandible. The cranium is the largest part of the skull enclosing the brain, eyes, ears and nose. The mandible is the lower jaw, also known as the jaw bone. A human adult skull is made of 22 bones: 8 cranial bones and 14 facial bones. Except for the mandible, where the bones meet are ragged edges called sutures.