THE NERVOUS SYSTEM 111 



Of these, the inner vascular layer corresponds to the pia mater 

 and arachnoid membrane of higher forms. The outer tougher 

 layer, the dura mater, consists of two sheets separated by an 

 interdural space filled with lymph. The roof of the diencephalon 

 in front of the pinealis, and also the roof of the medulla, is folded 

 in by the invasion of blood vessels of the pia mater to form the 

 anterior and posterior choroid plexuses, respectively. Such 

 choroid plexuses occur in vertebrates generally and are probably 

 the source of the cerebrospinal fluid, which fills the cavities of the 

 brain and spinal cord. In man this fluid may leave the brain 

 through minute passage ways in the roof of the medulla and join 

 the fluid in the meningial space about the pia mater, thus pro- 

 viding a means for circulating the fluid. The cerebrospinal fluid 

 is to be regarded as part of the circulatory mechanism of the 

 brain and cord. In addition it also serves as a protective 

 cushion of fluid. 



Cranial Nerves. — The brain is connected with different parts 

 of the head and adjacent regions by means of cranial nerves. 

 In Amphibia and lower vertebrates there are 10 pairs of such 

 nerves (Fig. 116), while the higher vertebrates (reptiles, birds, and 

 mammals) have two additional pairs, making 12 in all. Some 

 of the cranial nerves are composed of sensory fibers only, some of 

 motor, and others of both sensory and motor. These nerves are 

 known by name or by number, as follows: 



I. Olfactory nerve, sensory in function, arising as axons of 

 sensory cells located in the olfactory epithelium of the nose and 

 terminating in the olfactory lobe of the telencephalon. 



II. Optic nerve, sensory in function, arising in the retina of the 

 eye, from which it extends across the ventral surface of the 

 diencephalon in front of the pituitary gland to the optic lobes. 

 In all vertebrates except mammals there is a crossing over of the 

 fibers of the optic nerve at the optic chiasma, those from the 

 left eye going to the right side of the brain and vice versa. In 

 mammals the crossing is incomplete, some fibers remaining in 

 the side of origin. 



III. Oculomotor nerve, motor in function, arising from the ven- 

 tral surface of the mesencephalon and supplying four of the 

 extrinsic eye muscles: the superior, inferior, and internal recti 

 and the inferior oblique muscles. A branch also goes to the 

 ciliary ganglion of the eye. 



