AND PHYSIOLOGY. 



323 



Fig. 309. 



ter ; the gray or external, and the white or internal. This 

 structure is best seen when the brain is cut through in a hori- 

 zontal direction; the gray showing itself as an outside layer 

 with an irregularly scolloped edge, while the white is internal, 

 and constitutes the greater portion of the whole brain. 



574. Blood-Vessels of the Brain— Venous Sinuses, 

 ■ — The blood-vessels of the brain are very numerous, since one 

 sixth of all the blood is sent to this organ, although its weight 

 is about one fortieth of the body. The arteries are the most 

 numerous, and as already mentioned, those entering the head 

 from the front side of the neck, communicate very freely with 

 those coming in at the back side of the head, (see Fig. 213), 

 making a perfect circle of communication, so that there may 

 be no impediment to the cir- 

 culation of the blood through 

 the nervous center. The veins 

 are not numerous. Some are 

 found on the surface of the 

 brain, but only a few pene- 

 trate into its substance. But 

 large channels are found be- 

 tween the membranes called 

 Sinuses, which do not pre- 

 sent the ordinary characteris- 

 tics of veins, except that they 

 convey the blood to the heart. 

 The two sinuses which are of 

 the largest size, discharge 

 their contents into the jugu- 

 lar veins. (Fig. 309.) 



575. Membranes of the 



C C 1' e b TO S p i n a 1 Center. thalmie Veins. 2, Cavernous Sinus. 3, 



D.. n o TIT « + mi Circular. 4, 6, Inferior Petrosal. 5, 9, Oc- 



lira Mater. lhree mem- cipital Sinuses. T, InternalJugular Vein. 



574. What proportion of the blood goes to the brain ? Which are the most numerous, 

 the arteries or the veins ? What are the sinuses found there ? 



