142 THE HUMAN BODY 



spinal cord by narrow channels. The chambers are four in number, 

 and are known as the ventricles of the brain. There is one in each 

 cerebral hemisphere (p. 145). These are called the lateral ventricles. 

 Numerically they rank as first and second. The lateral ventricles 

 open into a narrow chamber in the base of the cerebrum in the 

 mid line, known as the third ventricle. This in turn communi- 

 cates by a narrow channel (the aqueduct of Sylvius) with the 

 cavity of the brain stem (p. 146) which is called the fourth ven- 

 tricle. 



The cavity of the fourth ventricle communicates with the 

 arachnoid space (p. 141) by three small openings in its roof, one in 

 the mid line and one at each lateral border. By these openings the 

 cerebrospinal fluid which occupies the arachnoid space is con- 

 tinuous with that which fills the ventricles and central canal. 



Cerebrospinal Fluid. This fluid, which occupies all the spaces 

 within and around the central nervous system, is in general similar 

 to the medium, lymph, by which the other tissues of the body are 

 bathed (p. 18). It represents some chemical differences, however, 

 which become accentuated in certain diseases. In the lumbar 

 region there is room between the processes of the vertebrae so that 

 a hypodermic needle can be thrust into the arachnoid space and 

 some of the cerebrospinal fluid withdrawn. The operation is 

 simple and by the application of cocaine to the skin made virtually 

 painless. Chemical examination of the fluid so obtained is often 

 helpful in diagnosing obscure complaints. Under certain diseased 

 conditions there is a great accumulation of cerebrospinal fluid. 

 The pressure of this upon the delicate nervous structures is likely 

 to do them harm, and "lumbar puncture" is often resorted to to 

 draw off the accumulated fluid and relieve the pressure. Some- 

 times in young children the accumulation of fluid distends the 

 head far beyond its normal size, giving the condition known as 

 " Jiydrocephalus." 



The Spinal Cord (Fig. 56) is nearly cylindrical in form, being 

 however a little wider from side to side than dorsiventrally, and 

 tapering off at its posterior end. Its average diameter is about 19 

 millimeters (f inch) and its length 0.43 meter (17 inches). It 

 weighs 42.5 grams (\\ ounces). There is no marked limit be- 

 tween the spinal cord and the brain, the one passing gradually into 

 the other (Fig. 62), but the cord is arbitrarily said to commence 



