312 THE CIRCULATION IN THE BLOOD-VESSELS [CIJ. XXII. 



nerves ; drawing the finger-nail across the skin causes an immediate 

 wheat, or at least a red mark that lasts a considerable time. At one 

 time this was considered characteristic of affections of the cerebral 

 meninges such as tubercular meningitis, and was consequently called 

 the " meningeal streak." It, however, occurs in a variety of patho- 

 logical conditions of the nervous system, both cerebral and spinal. 



In certain conditions which lead to angina pectoris the pain in 

 the heart is in part due to its being unable to overcome an immense 

 peripheral resistance, and the condition is relieved by the adminis- 

 tration of such drugs as amyl-nitrite or nitroglycerin, which relax the 

 vessels and cause universal blushing. 



Raynaud's disease is one in which there is a localised constriction 

 of the vessels which is so effectual as to entirely cut off the blood 

 supply to the capillary areas beyond, and if this lasts any considerable 

 time may lead to gangrene of the parts in question. 



Local Peculiarities of the Circulation. 



The most remarkable peculiarities attending the circulation of blood through 

 different organs are observed in the cases of the brain, erectile organs, lungs, liver, 

 spleen, and kidneys. 



In the Brain. The brain must always be supplied with blood, for otherwise im- 

 mediate loss of consciousness would follow. Hence, to render accidental oblitera- 

 tion almost impossible, four large arteries are supplied to the brain, and these anas- 

 tomose together in the circle of Willis. The two vertebral arteries are, moreover, 

 protected in bony canals. Two of the brain arteries can be tied in monkeys, and 

 three or even all four in dogs, without the production of serious symptoms. In the 

 last case enough blood reaches the brain by branches from the superior intercostal 

 arteries to the anterior spinal artery. The sudden obliteration of one carotid artery 

 in man may in some cases produce epileptiform spasms ; the sudden occlusion of 

 both occasions loss of consciousness. Uniformity of supply is further ensured by 

 the arrangement of the vessels in the pia mater, in which, previous to their distribu- 

 tion to the substance of the brain, the large arteries break up and divide into 

 innumerable minute branches ending in capillaries, which, after frequent communi- 

 cation with one another, enter the brain and carry into nearly every part of it uni- 

 form and equable streams of blood. The arteries are enveloped in a special 

 lymphatic sheath. 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 on one side by the bony cranium, they are not compres- 

 sible by any force which the fulness of the arteries might exercise through the sub- 

 stance of the brain ; nor do they admit of distension when the flow of venous blood 

 from the brain is obstructed. No valves are placed between the vertebral veins and 

 the vena cava ; the vertebral veins anastomose with the cerebral sinuses. Hence on 

 squeezing the thorax and abdomen, venous blood can be pressed from those 

 parts out of any opening made into the longitudinal sinus. Expiration acts in the 

 same way ; it raises the cerebral venous pressure ; if the skull wall is defective the 

 brain expands owing to the distension of its capillaries during the expiratory act. 

 The exposed brain also expands with each systole of the heart. Owing to the fact 

 that the brain lies enclosed in the cranium, the arterial pulse is transmitted through 

 the brain substance to the cerebral veins, and so the blood issues from these in pulses. 



Since the brain is enclosed in the rigid cranium the volume of blood in the 

 cerebral vessels cannot alter unless the volume of the other cranial con tents alters in 

 the opposite sense. 



These conditions of the brain and skull led Monro and Kellie many years ago 



