Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 301 



of the body (Fig. 16.17). Although the cerebral lobes are distinct and the 

 brain is definitely bilateral, there are great numbers of intercrossing fibers 

 that insure synchronous action of the parts. The left side of the brain controls 

 the right side of the body, and vice versa; the fibers from one side of the cord 

 and brain cross to the opposite side. The sensory axons (bearing impulses 

 to the brain) cross over in the brain, in the medulla or above; the motor 



OLFACTORY LOBE 



CEREBRAL 



HEMISPHERES 



PINEAL BODIES 

 OPTIC LOBES 



CEREBELLUM 

 MEDULLA 



SPINAL CORD 



PERCH 

 FISH 



FROG 

 AMPHIBIA 



ALLIGATOR 

 REPTILE 



PIGEON 

 BIRO 



CAT 

 MAMMAL 



Fig. 16.18. Five types of vertebrate brains. In the cat's brain the pineal body 

 and the optic lobes are present, but are hidden by the cerebral hemispheres. 

 (Courtesy, MacDougall and Hegner: Biology. New York, McGraw-Hill Book Co., 

 Inc., 1943.) 



axons (bearing impulses to the muscles) cross over in the midbrain, pons, 

 medulla and at various levels below it (Fig. 16.11). 



Meninges. The brain and cord are enclosed in three protective coverings 

 of connective tissue v*'ith spaces between them filled with cerebrospinal fluid 

 (Fig. 16.19). The innermost one, pia mater (tender mother) is very delicate 

 and carries many blood vessels. It is intimately associated with the arachnoid 

 layer so called because of its open spaces like a spider's web; these are filled 

 with cerebrospinal fluid. The outermost cover, dura mater (hard mother), 

 is made of tough connective tissue, contains many blood vessels, and adheres 

 tightly to the cranium and vertebrae. Meningitis is an inflammation of the 

 meninges, especially the pia mater and arachnoid. 



Cerebrospinal Fluid. The central canal and the ventricles are continually 

 moistened by the cerebrospinal fluid. Most of this is formed by vascular 

 glands, the choroid plexuses, located in the ventricles. Much of the fluid 

 makes its way through holes in the choroid plexus of the fourth ventricle and 

 enters the space between the two delicate coverings of the brain and cord 

 so that these organs are actually surrounded by a blanket of fluid (Fig. 16.20). 

 It is produced more or less continuously and the excess is drained off through 

 hollow, button-shaped projections (villi) that dip into open lakes of venous 

 blood in the dura mater. The cerebrospinal fluid diffuses into the blood 

 through the thin caps of the villi whenever its pressure is higher than that of 

 the venous blood. 



