Biology of Man 503 



The cerebrum, which is the largest and most prominent part of the brain, 

 is divided into the right and left cerebral hemispheres. Each hemisphere 

 is divided by sulci into five distinct areas known as lobes (frontal, parietal, 

 temporal, occipital lobes, and the insula, the latter not visible from the 

 surface). The outer layer of the cerebrum, known as the cortex, has 

 numerous foldlike convolutions which greatly increase the surface area. 

 Certain functions are localized in specific regions of the cerebral cortex 

 as illustrated by the following: motor area, sensory areas (heat, cold, 

 pain, touch, light pressure, muscle sense), auditory area, visual area, 

 olfactory area (taste and smell), and speech area. Beneath the gray cor- 

 tex of the cerebrum is a mass of nervous tissue known as white matter. 



Spinal Nerves of Man 



Cervical (neck) 8 pairs 



Thoracic (thorax) 12 pairs 



Lumbar (back) 5 pairs 



Sacral (pelvis) 5 pairs 



Coccygeal (tail) 1 pair 



31 pairs 



The brain contains cavities (ventricles) as follows: fl) two lateral 

 ventricles, one in each cerebral hemisphere; (2) the third ventricle be- 

 hind the lateral ventricles and connected with each by an opening called 

 the foramen of Monro; the fourth ventricle in front of the cerebellum 

 and behind the pons and medulla, being connected with the third ven- 

 tricle by a small canal called the aqueduct of Sylvius. The coverings of 

 the brain are called meninges and are the same as for the spinal cord 

 (dura mater, outer; arachnoid, middle layer; pia mater, inner). Thin 

 layers of fluid separate the various layers. 



Functions of the cerebrum in addition to those already mentioned are 

 as follows: it governs all our mental activities (reason, will, memory, 

 intelligence, higher feelings, and emotions) ; it is the seat of consciousness, 

 interpreter of sensations, originator of voluntary acts; it is a control on 

 many reflex acts which originate as involuntary (weeping, laughing, 

 defecation, micturition, etc.). 



The cerebellum (Figs. 246 and 247) lies at the base or posterior part 

 of the brain. The outer, cerebellar cortex is made of gray matter, which 

 is not convoluted but is traversed by numerous furrows (sulci). All 

 functions of the cerebellum are below the level of consciousness, the 

 main function being the reflex control of skeletal muscle activities. 



The midbraiii connects the cerebral hemispheres with the cerebellum 

 and pons. Two pairs of round elevations, known as the corpora quad- 



