THE BRAIN. 121 



Tentorium Cerebelli, a lamina of dura mater supporting the posterior lobes of 

 the brain, and covering the upper surface of the cerebellum. It incloses 

 the lateral and superior petrosal sinuses. 

 Falx Cerebelli, projects between the lateral lobes of the cerebellum, from the 



tentorium to the foramen magnum. 



Pacchionian Bodies, clusters of white granulations situated on the outer 

 and inner surfaces of the dura mater, in the superior longitudinal sinus, 

 and on the pia mater, found only after the 7th year, and of unknown 

 function. 



Describe the Arachnoid. It is the serous sac which forms the middle 

 membrane, having a visceral and a parietal layer, the latter being reflected 

 over the interior surface of the dura mater. The Anterior Sub-arachnoid Space 

 is the interval between it and the pia mater of the brain at the base, where it 

 is extended across between the two middle lobes. The Posterior Sub-arachnoid 

 Space is a similar interval between the hemispheres of the brain and the me- 

 dulla oblongata. These spaces communicate together across the crura cere- 

 bri ; and also with the general ventricular cavity, by an opening in the lower 

 boundary of the 4th ventricle ; and contain the cerebro-spinal fluid which forms 

 a water-bed for the nervous centres. The sac of the arachnoid also contains a 

 serous fluid in small quantity. 



What is the Pia Mater ? It is the vascular membrane, being supplied by 

 the branches of the internal carotid and vertebral arteries. It covers the sur- 

 face of the brain, dipping down into all the sulci, and forms the velum inter- 

 positum and choroid plexus of the 4th ventricle ; and contains lymphatics and 

 nerves. 



How is the Brain divided ? Into the cerebrum, cerebellum, pons Varolii, 

 and medulla oblongata. Its average weight in the male adult is 49^ oz., in 

 the female 44 oz., of which the cerebrum is about seven-eighths. The ex- 

 treme weights in 278 male cases were 65 oz. and 34 oz., in 191 female cases, 

 56 oz. and 31 oz. 



Describe the principal Lobes and Fissures of the Cerebrum. Each lateral 

 half of the cerebrum, or hemisphere, has the following 5 lobes and 8 fissures, 

 besides many of less importance. 



Frontal Lobe, bounded internally by the longitudinal fissure, below by the 

 fissure of Sylvius, and posteriorly by the fissure of Rolando. 



Parietal Lobe, extending down to the fissure of Sylvius, and antero-poste- 

 riorly from the fissure of Rolando to the parieto-occipital. 



Occipital Lobe, behind the parieto-occipital fissure. 



Temporo-sphenoidal Lobe, lying in the middle fossa of the skull, and bounded 

 in front by the fissure of Sylvius. 

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