It is formed of a number of long, thin, whitish threads called nerves extending between central nervous system and body tissues. Each nerve is formed of bundles of nerve fibres, fasciculi, held together by connective tissue and surrounded by a white fibrous connective tissue sheath called epineurium.
The nerve fibres are classified into two categories on the basis of presence or absence of myelin (white fatty) sheath.
(a) Medullated or Myelinated nerve fibres.
(b) Non-medullated nerve fibres.

On the basis of their origin, nerves are of two types
(a) Cranial or cerebral nerves which either arise from or end into brain.
(b) Spinal nerves which arise from spinal cord.
(a) Cranial nerves
(1) 10 pairs of cranial nerves are present in an anamniote (fishes and amphibians).
(2) Number of cranial nerves found in frog is ten pairs (20).
(3) 12 pairs of cranial nervers are present in an amniote (reptiles, birds and mammals).
(4) Number of cranial nerves found in rabbit and man is 12 pairs (24).
(5) The first 10 pairs are common for frog and rabbit. The additional pairs found in rabbit are spinal accessory and hypoglossal.
(6) The smallest cranial nerbe is trochlear in human beings, but all animals smallest cranial nerve is abducens.
(7) The largest cranial nerve is trigeminal in human beings but vagus is largest cranial nerve in all animals.
(8) Vagus supplies the regions other than head.
(9) The sensory cranial nerves are
I Olfactory – Smell
II Optic – Vision
VIII Auditory – Hearing and equilibrium
(10) The motor cranial nerves are : III, IV, VI, XI and XII.
(11) Extraocular muscle nerves are : III, IV and VI.
(12) The mixed cranial nerves are : V, VII, IX and X (4 pairs).
(b) Spinal nerves : Spinal nerves arise from gray matter of spinal cord. There are 31 pairs of spinal nerves in man (37 pairs in rabbit). All spinal nerves are mixed. The spinal nerves in man are divided into 5 groups.
(1) Cervical (C)  8 pairs –– in Neck region
(2) Thoracic (T)  12 pairs –– in thoracic region
(3) Lumbar (L)  05 pairs –– upper part of abdomen
(4) Sacral (S)  05 pairs –– lower part of abdomen
(5) Coccygeal (CO)  01 pairs –– represent the tail nerves
31 pairs
Number of spinal nerves in frog is 10 pairs. In some frog like Rana tigrina, 10th pair may reduced or absent. The first pair of spinal nerves in frog is hypoglossal. The last pair of cranial nerves of mammals has the same name. Brachial plexus is formed by 2nd and 3rd spinal nerves in frog. Sciatic plexus is formed by 7, 8 and 9 spinal nerves in frog. Glands of Swammerdam are calcareous glands found at the places of emerging of spinal nerves in frog.
Spinal nerve formula can be written as : C8, T12, L5, S5, CO1, Spinal nerves exit via intervertebral foramen. Each spinal nerve arises from spinal cord by 2 roots
(1) Dorsal (= Afferent = Sensory = Posterior) root is a continuation of dorsal horn and is formed of gray matter. It presents a ganglionic swelling in middle, called dorsal root ganglion. These transmit sensory nerve impulses from the sense organs to spinal cord (touch, pain, temperature). They activate involuntary reflexes.
(2) Ventral (= Efferent = Motor) root are continuation of ventral root and is also formed of gray matter. No ganglion are present. It is formed of only efferent nerve filers. They transmit motor nerve impulses to effector organs e.g., glands and muscles. Each spinal nerve has 3 branches –
(i) Ramus dorsalis : Supplies to skin and muscles of dorsal side.
(ii) Ramus ventralis : Supplies to skin and muscles of ventral and lateral sides and also to upper and lower limbs. Ventral root of certain spinal nerve form 5 nerve plexi on either side, i.e., cervical, thoracic, lumber, sacral, caudd.
(iii) Ramus communicans : It joins sympathetic ganglion of autonomic nervous system.
(iv) Autonomic nervous system : As mentioned before, the visceral part of peripheral nervous system regulates and co-ordinates the activities of internal or visceral organs. Autonomic nervous system was discovered by Langley. Autonomic nervous system (ANS) automatically regulates the activities of smooth muscles, cardiac muscles and glands. This co-ordination is involuntary. Autonomic nervous system usually operates without conscious control. Autonomic nervous system is entirely motor. All autonomic axons are efferent fibres. Efferent neurons are preganglionic with myelinated fibres and postganglionic with unmyelinated fibres. Autonomic nervous system is regulated by centres in brain like cerebral cortex, hypothalamus and medulla oblongata. Autonomic fibres release chemical transmitters at synapse. On the basis of the transmitter produced, these fibres may be classified as cholinergic or adrenergic. Cholinergic fibres release acetylcholine. Adrenergic fibres produce norepinephrine (noradrenaline), also called sympathetin.
Nature of autonomic control : The autonomic nervous system regulates and co-ordinates such vital involuntary activities like heart beat, breathing, maintenance of the composition of body fluids (= homeostasis) and body temperature, gut peristalsis, secretion of glands, etc. For this control, a visceral organ is normally innervated by two, instead of only one, postganglionic fibres having antagonistic excitatory effects; while one fibre promotes the activity of an organ, the other fibre retards it.
Autonomic nervous system consists of two divisions
(a) Sympathetic (= Thoracolumbar out flow)
(b) Parasympathetic (= Cranio-sacral out flow)

# (a) Sympathetic ANS
 Thoraco Lumber out flow (all thorocic + 3 lumber)
 Preganglionic nerve small.
 Post ganglionic nerve long.
 Preganglionic nerve secrete acetyl choline.
 Postganglionic nerve secrete sympathatin. (nor-epinephrine)
 It shows sympathy (generally increase the function).
 Expenditure of energy takes place.
 It increase defence system of body against adverse condition.
 It is active in stress condition, pain, fear and anger.

# (b) Parasympathatic
 ANS Cranio sacral outflow (cranial-III, VII, IX, X Nerves)-(sacral-II, III, IV Nerves)
 Preganglionic nerve long.
 Postganglionic nerve small.
 Secrete acetyl choline only.
 It provide relaxation, comfort, pleasure, at the time of rest.
 Restoration and conservation of energy takes place.
 Collateral ganglia present in sympathetic nervous system.
 Horner’s syndrome results from the damage of sympathetic trunk of one side.
 A patient of Horner’s syndrome exhibits lack of sweating (on affected side), sunken eyes and constricted pupil.