OH. XXI.] LOCAL PECULIARITIES OF THE CIRCULATION. 2/7 



of valves prevents regurgitation, so the entire effect of the con- 

 tractions is auxiliary to the onward current of blood. Analogous 

 phenomena have been observed in other animals. 



A venous pulse is observed under the conditions previously 

 described (p. 274) when the arterioles are dilated so that the 

 arterial pulse passes through the capillaries to the veins. 



A venous pulse is also seen in the superior and inferior vena 

 cava near to their entrance into the heart ; this corresponds to 

 variations of the pressure in the right auricle. When the 

 ventricle is contracting there is a slow rise due to the fact that 

 the blood cannot get into the ventricle and so distends the 

 auricle ; a second short sharp elevation of pressure is produced by 

 the auricular systole. Alterations of venous pressure are also 

 produced in the great veins by the respiratory movements, the 

 pressure sinking during inspiration, and rising during expiration. 



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 immediate loss of consciousness would follow. 

 Hence, to render accidental obliteration almost impossible, four 

 large arteries are supplied to the brain, and these anastomose 

 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 distribution to the substance of the 

 brain, the large arteries break up and divide into innumerable 

 minute branches ending in capillaries, which, after frequent 

 communication with one another, enter the brain and carry into 

 nearly every part of it uniform 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 



