Nervous System and Sense Organs 201 



difficult due to the lack of control of the leg muscles. Unlike the cere- 

 brum, the control exercised by the cerebellum is on the same side of 

 the body, that is, the left side of the cerebellum controls the coordina- 

 tion of the voluntary muscles of the left side of the body. 



Ventrally the lobes of the cerebellum are connected by the trans- 

 verse fibers of the pons. Other longitudinal fibers in this same struc- 

 ture connect the myelencephalon with the cerebrum. 



Only one structure form the myelencephalon: the medulla oblon- 

 gata. This functions as a connecting pathway between the spinal cord 

 and the rest of the brain. All the impulses passing between the brain 

 and the cord must go through the myelencephalon. In addition, there 

 are nerve centers which control many reflex activities such as rate of 

 respiration, rate of heart beat, digestion, and vasoconstriction and dila- 

 tion. 



As has been indicated, all parts of the brain are connected with 

 one another through an intricate arrangement of nerve tracts. Those 

 fiber tracts, such as the corpus callosum, pons, and many others, which 

 cross the midline of the brain to connect the lateral lobes, are called 

 commissures. In addition to the commissures there are longitudinal 

 nerve tracts which run along the base of the brain to connect the vari- 

 ous regions. 



The interior of the entire central nervous system of vertebrates is 

 hollow. Within the brain, these spaces are much reduced in size and 

 are present simply as a series of four ventricles (Fig. 62). The first 

 two of these ventricles are in the cerebrum, one in each hemisphere. 

 These connect with the third ventricle in the diencephalon. A narrow 

 passageway, the aqueduct, in the mesencephalon connects the third ven- 

 tricle with the fourth in the myelencephalon. The fourth ventricle con- 

 nects directly with the hollow center of the cord. Over the roof of the 

 fourth ventricle is a thin vascular membranous wall, the choroid 

 plexus. 



The ventricles as well as the hollow center of the cord are filled 

 with a fluid, the cerebrospinal fluid, which is derived from the blood 

 in the choroid plexus. This fluid serves to keep the nervous tissues 

 moist, as a medium of exchange for food, oxygen, and waste materials, 

 and to equalize pressures within the central nervous system. 



Surrounding the brain and spinal cord are three protective layers 

 of tissue collectively known as the meninges. The inner-most of these 

 is the vascular pia mater which follows the folds and dips of the brain 

 and cord very closely. The next layer is the arachnoid, a thin and 



