Atrium (heart) of blood occurs. In birds, mammals, and some other reptiles (alligators in particular) the partitioning of both chambers is complete. Atrium (heart) The atrium is the upper chamber in which blood enters the heart. There are two atria in the human heart – the left atrium connected to the lungs, and the right atrium connected to the venous circulation. The atria receive blood, and when the heart muscle contracts they pump blood to the ventricles. All animals with a closed circulatory system have at least one atrium/auricle (humans have two atria). The atrium used to be called the "auricle", and
Lateral ventricles lateral, third, and fourth ventricles, connected by thinner channels. In the lateral ventricles, specialized areas – choroid plexuses – appear, which produce cerebrospinal fluid. If its production is bigger than reabsorption or its circulation is blocked – the enlargement of the ventricles may appear and cause a hydrocephalus. The neural canal that does not expand and remains the same at the level of the midbrain superior to the fourth ventricle forms the cerebral aqueduct. The fourth ventricle narrows at the obex (in the caudal medulla), to become the central canal of the spinal cord. Fetal lateral ventricles may be diagnosed