SENSE ORGANS AND NERVOUS COORDINATION 



599 



connective tissue, the meninges, that encase the brain and spinal cord. 

 The innermost meninx, the pia mater, is a very vascular membrane that 

 is closely applied to the surface ot the brain and spinal cord. Certain 

 parts of it help to form the choroid plexuses. A delicate arachnoid mem- 

 brane lies peripheral to the pia, and a very tough dura mater forms a 

 protective envelope around the entire central nervous system. The cere- 

 brospinal fluid lies in the space between the arachnoid and pia. It is 

 produced continuously and reenters the circulatory system by filtering 

 into certain venous sinuses located in the dura mater covering the brain. 

 The cerebrospinal fluid forms a protective liquid cushion about the 

 brain and spinal cord, and also helps to nourish the tissue of the central 

 nervous system. 



Medulla Oblongata. Brain functions are exceedingly complex, 

 and far from completely understood. The medulla oblongata (Fig. 29.14) 

 lies between the spinal cord and the rest of the brain and is funda- 

 mentally the same in all vertebrates. The gray columns of the spinal 

 cord extend into the medulla, but within the brain they become dis- 

 continuous, breaking up into discrete islands of cell bodies known as 

 nuclei. The dorsal nuclei receive the afferent neurons from cranial 

 nerves that are attached to this region, and contain the cell bodies of 



Venous sinus 

 in dura, maler 



Cerebral veins 



Sulcus of Rolando 

 rFornix 



Corpus 

 callosu 



Lateral 

 ventricL 



Foramen 

 of Monro 



Ventricle EL 



i^racTnnoid villus 

 ater 



"Arachnoid. 



embrane 



Dura 

 mater 



Epilhalcunus 



Pinecd 

 body 



Colliculi 



Cerebellum 

 Ventricle ]Z 



Cerebral aqueduct 



Medulla oblongata 



Central canal- 

 Spinal cord 



Figure 29.14. A sagittal section of the human brain and its surrounding meninges. 

 Cerebrospinal fluid is produced by the choroid plexuses, circulates as indicated by the 

 arrows, and finally enters a venous sinus in the dura mater. (Modified after Rasmussen.) 



