510 NERVOUS SYSTEM. 



third, sixth, seventh, eighth, and spinal accessory. They accompany the branches 

 of the arteries. 



THE BRAIN. 



The "brain (encephalon) is that portion of the cerebro-spinal axis that is cor- 

 tained in the cranial cavity. It is divided into four principal parts, viz : the 

 cerebrum, the cerebellum, the pons Yarolii, and medulla oblongata. 



The cerebrum forms the largest portion of the encephalic mass, and occupies a 

 considerable part of the cavity of the cranium, resting in the anterior and middle 

 fossas of the base of the skull, and separated posteriorly from the cerebellum by 

 the tentorium cerebelli. About the middle of its under surface, is a narrow con- 

 stricted portion, part of which, the crura cerebri, is continued onwards into the 

 pons Varolii below, and through it to the medulla oblongata and spinal cord ; 

 whilst another portion, the crura cerebelli, passes down into the cerebellum. 



The cerebellum (little brain or after-brain) is situated in the inferior occipital 

 fossae, being separated from the under surface of the posterior lobes of the ceje- 

 brum by the tentorium cerebelli. It is connected to the rest of the encephalic 

 mass by means of connecting bands, called crura ; of these, two ascend to the 

 cerebrum, two descend to the medulla oblongata, and two blend together in front, 

 forming the pons Varolii. 



The pons Varolii is that portion of the encephalic mass which rests upon the 

 upper part of the basilar process. It constitutes the bond of union of the various 

 segments above-named, receiving, above, the crura from the cerebrum ; at the 

 sides, the crura from the cerebellum; and being connected, below, with the 

 medulla oblongata. 



The medulla oblongata extends from the lower border of the pons Varolii to 

 the upper part of the spinal cord. It lies beneath the cerebellum, resting on the 

 lower part of the basilar groove of the occipital bone. 



Weight of the encephalon. The average weight of the brain, in the adult male, 

 is 49 \ oz., or a little more than 3 Ibs. avoirdupois; that of the female, 44 oz.; the 

 average difference between the two being from 5 to 6 oz. The prevailing weight 

 of the brain, in the male, ranges between 46 oz. and 53 oz. ; and in the female, 

 between 41 oz. and 47 oz. In the male, the maximum weight out of 278 cases 

 was 65 oz., and the minimum weight 34 oz. The maximum weight of the adult 

 female brain, out of 191 cases, was 56 oz., and the minimum weight 31 oz. It 

 appears that the weight of the brain increases rapidly up to the seventh year, 

 more slowly to the period between sixteen and twenty, and still more slowly to 

 that between thirty and forty, when it reaches its maximum. Beyond this period, 

 as age advances and the mental faculties decline, the brain diminishes slowly in 

 weight, about an ounce for each subsequent decennial period. These results apply 

 alike to both sexes. 



The size of the brain appears to bear a general relation to the intellectual 

 capacity of the individual. Cuvier's brain weighed rather more than 64 oz., that 

 of the late Dr. Abercrombie 63 oz., and that of Dupuytren 62 \ oz. On the other 

 hand, the brain of an idiot seldom weighs more than 23 oz. 



The human brain is heavier than that of all the lower animals excepting 

 the elephant and whale. The brain of the former weighs from 8 Ibs. to 10 Ibs.; 

 and that of the whale, in a specimen seventy-five feet long, weighed rather more 

 than 5 Ibs. 



MEDULLA OBLONGATA. 



The medulla oblongata is the upper enlarged part of the spinal cord, and extends 

 Crom the upper border of the atlas to the lower border of the pons Varolii. It is 

 directed obliquely downwards and backwards, its anterior surface resting on the 

 basilar groove of the occipital bone, its posterior surface being received into the 

 fossa between the hemispheres of the cerebellum, forming the floor of the fourth 



