The =interventricular foramen= (or =foramen of Monroe=).--If the choroid plexus of the lateral ventricle (Fig. 148, _e_) is followed to its medial end, it is found to pass through a foramen (at _h_) in which it becomes continuous with the opposite plexus or roof of the third ventricle. This foramen leads from one lateral ventricle to the other and is connected ventrally by a median opening with the third ventricle. It is thus Y-shaped or T-shaped and is the interventricular foramen (or foramen of Monroe). The foramen lies opposite the middle of the corpus striatum and caudad of the pillars of the fornix. =Membranes of the Brain.=--The membranes of the brain are the =dura mater=, the =arachnoid=, and the =pia mater=. The dura mater is a strong fibrous membrane lining the cranial cavity and covering the brain. It is strongly attached to the projections of the base of the skull and to the tentorium. It dips between the cerebral hemispheres and olfactory bulbs, forming the =falx cerebri=. It likewise dips between the cerebrum and the cerebellum, covering both surfaces of the bony tentorium. The dura mater does not pass into the fissures or sulci of the surface of the brain. It becomes continuous with the sheath of the cranial nerves as they leave the skull. It is continuous with the dura mater of the spinal cord. The arachnoid and pia mater are essentially similar to the same structures on the spinal cord. The pia mater dips into the fissures and sulci of the brain. II. THE PERIPHERAL NERVOUS SYSTEM. 1. =Cranial Nerves. Nervi cerebrales.=--I. N. OLFACTORIUS.--The fasciculi of the olfactory nerves arise from the olfactory bulb (Fig. 144, _a_) and pass through the foramina of the cribriform plate, upon which the bulb lies, to be distributed to the olfactory mucous membrane of the nose. II. N. OPTICUS.--The second nerve or optic (Fig. 138, _II_) arises from the optic chiasma (Fig. 138, _c_), passes through the optic foramen, and extends in an S-shaped curve to the eyeball. Its course is craniodorsad. It pierces the sclerotic and choroid coats of the eye and spreads out into the retina.