motor fibres, and dendrites in case of afferent sensory fibres of the nerves), along with a variety of membranes that wrap around and segregate them into nerve fascicles. The vertebrate nervous system is divided into the central and peripheral nervous systems. The central nervous system (CNS) consists of the brain, retina, and spinal cord, while the peripheral nervous system (PNS) is made up of all the nerves and ganglia (packets of peripheral neurons) outside of the CNS that connect it to the rest of the body. The PNS is further subdivided into the somatic and autonomic nervous systems. The somatic nervous system is made up of "afferent" neurons, which bring sensory information from the somatic (body) sense organs to the CNS, and "efferent" neurons, which carry motor instructions out to the voluntary muscles of the body. The autonomic nervous system can work with or without the control of the CNS (that's why it's called 'autonomous'), and also has two subdivisions, called sympathetic and parasympathetic, which are important for transmitting motor orders to the body's basic internal organs, thus controlling functions such as heartbeat, breathing, digestion, and salivation. Autonomic nerves, unlike somatic nerves, contain only efferent fibers. Sensory signals coming from the viscera course into the CNS through the somatic sensory nerves (e.g., visceral pain), or through some particular cranial nerves (e.g., chemosensitive or mechanic signals). Orientation in neuroanatomy In anatomy in general and neuroanatomy in particular, several sets of topographic terms are used to denote orientation and location, which are generally referred to the body or brain axis (see Anatomical terms of location). The axis of the CNS is often wrongly assumed to be more or less straight, but it actually shows always two ventral flexures (cervical and cephalic flexures) and a dorsal flexure (pontine flexure), all due to differential growth during embryogenesis. The pairs of terms used most commonly in neuroanatomy are: Dorsal and ventral: dorsal loosely refers to the top or upper side, represented by the roof plate of the brain, and ventral to the bottom or lower side, represented by the floor plate. These descriptors originally were used for dorsum and ventrum – back and belly – of the body; the belly of most animals is oriented towards the ground; the erect posture of humans places our ventral aspect anteriorly, and the dorsal aspect becomes posterior. The case of the head and the brain is peculiar, since