Chap. XIX] THE NERVOUS SYSTEM 385 



spinal cord, besides being, as it were, a continuation of the gray- 

 matter in the centre of the cord. It is also connected at its 

 upper end by numberless fibres to the gray matter on the surface 

 of the cerebrum. 



Weight of the brain. — With the exception of the whale and the 

 elephant, the human brain is heavier than that of any of the lower 

 animals. The average weight of the brain in the male is forty-nine 

 and a half ounces (1403 grams) ; in the female, forty-four 

 ounces (1247 grams). It appears that the weight of the brain 

 increases rapidly up to the seventh year, more slowly to between 

 sixteen and twenty, and still more slowly to between thirty and 

 forty, when it reaches its maximum weight. Beyond this age the 

 brain diminishes slowly in weight, about an ounce every ten years. 



Divisions of the brain. — The brain is divided into four principal 

 parts : the cerebrum, the cerebellum, the pons Varolii, and the 

 medulla oblongata. 



The medulla oblongata. — The medulla oblongata, also known as 

 the spinal bulb, is continuous with the spinal cord, which, on passing 

 into the cranial cavity through the foramen magnum, widens into 

 an oblong-shaped mass. It is directed backward and downward, 

 its anterior surface resting on a groove in the occipital bone, and 

 its posterior surface forming the floor of a cavity between the 

 two halves, or hemispheres, of the cerebellum. It is hollow and the 

 cavity, called the fourth ventricle, is an expanded continuation 

 of the tiny central canal which runs throughout the whole length 

 of the spinal cord. The gray matter is found in the interior, and 

 the white matter on the exterior ; most of the gray matter is found 

 on the floor of the fourth ventricle, and from this gray matter arise 

 most of the cranial nerves. The medulla has an anterior and a 

 posterior median fissure ; at the lower part of the anterior fissure 

 are nerve-fibres which cross from one side to the other or decussate. 



Functions of the medulla oblongata. — The functions of the 

 medulla are similar to the first three listed under the functions of 

 the cord, i.e. conduction, reflex action, and automatic action. 



As all the impressions passing between the brain and spinal cord 

 must be transmitted through the medulla, the function of con- 

 duction is a very important one. As previously stated, the 

 medulla contains important vital and reflex centres. The prin- 

 cipal ones are : — 

 2c 



