242 THE CIRCULATION OF THE BLOOD 



been followed satisfactorily in many of the important parts of the body, 

 though the supply for some regions is yet obscure. This is particularly 

 true for the brain, where such supply is apparently absent. The two groups 

 of fibers run the same course, except in the cephalic and sacral regions already 

 mentioned. They may, therefore, be described together. 



The Vascular Nerve Supply for the Head. The vascular nerves 

 for the head, face, and mouth have their origin in the cord from the first to 

 the fifth dorsal spinal nerves. They pass through the white rami to sym- 

 pathetic ganglia, through the stellate ganglion, and up the cervical sympa- 

 thetic nerve to the superior cervical ganglion. From this ganglion they run 

 to their distribution, either along with the arteries, as with the salivary sup- 

 ply, or with the sensory nerves, as in the nerves to the mucous membrane 

 of the mouth, etc. The vascular nerves supplied to the base of the ear follow 

 the above course, but the nerves for the tip leave the stellate ganglion in the 

 ramus vertebralis, run to the third cervical nerve, and pass with its auricular 

 branch to the ear, a circuitous route determined by Fletcher. 



The great exception to the above origin is with the vaso-dilator group. 

 Dilator fibers leave the base of the brain in the direct path of the seventh 

 cranial nerve to supply the submaxillary and sublingual glands, in the ninth 

 cranial nerve to the parotid gland, and in both these to the tongue. 



The Vascular Regulation in the Brain. The brain requires a large 

 and uniform supply of blood for the due performance of its functions. This 

 object is effected through the number and size of its arteries, the two internal 

 carotids and the two vertebrals. It is also desirable that the force with 

 which this blood is sent to the brain should be subject to less variation from 

 external circumstances than it is in other parts, an effect that is accomplished 

 by the free anastomoses of the large arteries in the circle of Willis. This 

 arrangement insures that the supply of blood will be uniform in both hemi- 

 spheres even though it may be limited through operation or accident to one 

 or more of the four principal arteries. Uniformity of supply is further 

 insured by the arrangement of the vessels in the pia mater. Previous to 

 their distribution to the substance of the brain the large arteries break up 

 and divide into innumerable minute branches. These capillaries after 

 frequent communication with one another enter the brain in a very uniform 

 and equable distribution. The arrangement of the veins within the cranium 

 is also peculiar. The large venous trunks or sinuses are formed so as to be 

 scarcely capable of change of size; and composed, as they are, of the tough 

 tissue of the dura mater, and in some instances bounded by the bony cranium, 

 they are not compressible by any force which the fullness of the arteries 

 might exercise through the substance of the brain. Nor do they admit of 

 distention when the flow of venous blood from the brain is obstructed. 



The mechanical conditions in the brain and skull formerly appeared 

 enough to justify the opinion that the quantity of blood in the brain must 



