716 



DISSECTION OF THE BRAIN. 



Outline of 



cranial 



mass. 



Medulla 

 oblongata. 



Pons Varolii 



and its 

 rouuections. 



Cerebellum. 



Cerebrum, 



and its great 

 divisions. 



Weight of 

 brain. 



Three 

 membranes. 



Dura mater 



Arachnoid 

 membrane : 



relations 

 to sulci ; 



SUBDIVISIONS OF THE ENCEPHALON. Before the description of 

 the membranes and vessels is given, the chief subdivisions of the 

 encephalon may be shortly noticed. 



The cranial or encephalic mass of the nervous system (fig. 268, 

 p. 741), consists of cerebrum or great brain, cerebellum or small brain, 

 pons, and medulla oblongata. Each of these parts has the following 

 situation and subdivisions : 



The medulla oblongata, or bulb of the spinal cord (fig. 268, a), lies 

 in the groove between the halves of the cerebellum, and is divided 

 into two symmetrical parts by a median fissure. To it several of the 

 cranial nerves are united. 



The pons Varolii (d) is situate above the medulla oblongata, 

 and is marked along the middle by a groove, which indicates a 

 separation into halves and which lodges the basilar artery. Above it 

 are two large processes (crura cerebri, /) connecting it to the 

 cerebrum ; and on each side it is united to the cerebellum by a 

 similar white mass. 



The cerebellum (6), or the small brain, is separated into two 

 hemispheres by a median groove ; and its surface is marked by 

 concentric lamina. 



The cerebrum (r and p) t or the large brain, is divided into two 

 hemispheres by a longitudinal fissure in the middle line ; and each 

 hemisphere presents a deep transverse cleft the fissure of Sylvius. 

 The surface of the hemispheres is convoluted. 



The average weight of the brain in the European male is about 

 49 oz. ; in the female about 44 oz. 



MEMBRANES OF THE BRAIN. The coverings of the brain (meninges) 

 are three in number, viz., dura mater, arachnoid, and pia mater. The 

 dura mater is a firm fibrous investment, which separates and supports 

 the different parts of the brain, and serves as an internal periosteum to 

 the cranial bones. The pia mater is the most internal layer ; it is 

 adherent to the brain substance and contains the ramifications of the 

 vessels of the brain. The arachnoid, which is interposed between 

 the other two, is the membrane that is seen when the brain is 

 removed from the cranial cavity. 



Besides enveloping the brain, these membranes are prolonged on 

 the cord into the spinal canal. Only the cranial part of the last 

 two will be now noticed. For the description of the cranial portion 

 of the dura mater, see pp. 507 et seq. 



The ARACHNOID is a very thin fibrous membrane, which envelopes 

 the brain loosely, and is separated from the dura mater by the 

 interval named the sultdural tpace and from the pia mater by the sub- 

 arachnoid space. Its outer surface is free and smooth and in the 

 natural state is in close apposition to the dura mater. The inner 

 surface is attached to the pia mater by numerous fine cords and 

 bands, which cross the subarachnoid space. The membrane covers 

 the convolutions and laminae of the large and small brain, bridging 

 over the sulci between them, and at the under surface or base of the 

 brain it stretches across from side to side between the cerebral 



