678 



CIRCULATION. 



reason to think that the motion of the Mood 

 is first arrested in the pulmonary capillaries. 



The state of our knowledge does not, it must 

 be confessed, permit us to offer a satisfactory 

 explanation of the cause of the above-men- 

 tioned phenomena. We have already stated 

 reasons against regarding the stagnation of the 

 blood in the lungs in asphyxia as attributable 

 to a loss of the supposed vital power of motion 

 belonging to the blood in the capillary vessels: 

 and we think it quite as just to regard the stag- 

 nation as the effect of over-stimulation and 

 constriction of the minute vessels of the lungs 

 by the dark blood, as to attribute it, in the 

 manner some have done, to the deficiency of 

 that stimulation which arterial blood, without 

 any good reason, is presumed by them to give 

 to the small vessels. 



2. Circulation within the cranium. The 

 limits of this essay do not permit us to do more 

 than allude very shortly to the nature of the 

 circulation within the cranium, a subject, in 

 some respects, nearly related to the facts just 

 stated, and of great importance from the general 

 dependence of the state of the cerebral func- 

 tions upon the quantity and force of blood which 

 flows through the brain. 



The bloodvessels within the cranium are dif- 

 ferently situated from those in other parts of 

 the body in this respect, that they are removed 

 from the influence of atmospheric pressure. In 

 consequence of the unyielding nature of the 

 skull, and its being closed on all sides, except- 

 ing at the places where the nerves and blood- 

 vessels pass through the bones, the cavity of the 

 skull must necessarily be equally full at all 

 times ; and the spinal canal is in the same pre- 

 dicament. 



The whole quantity of fluid or solid matter, 

 then, within the cavity of the cranium and 

 spinal canal must be always the same ; or, 

 during the circulation just as much blood must 

 issue as enters it, and it is physically impossible 

 to increase or diminish the whole quantity con- 

 tained in the brain by increased pressure, by 

 opening of an artery or vein or any other means. 

 It was shewn by various well devised experi- 

 ments performed by the late Dr. Kellie,* that 

 in animals bled to death, while the rest of the 

 body was exsangueous, the brain retained its 

 usual appearance so long as the vault of the 

 cranium was entire, but that a perforation of the 

 skull, such as to allow the atmospheric pressure 

 to act upon the brain and bloodvessels of the 

 head, caused the evacuation of blood from the 

 head as from other parts of the body. 



While the whole bulk of the contents of the 

 cranium, however, must necessarily remain the 

 same, yet the relative quantity of arterial and 

 venous blood may vary within a short space of 

 time, the pressure exerted by the blood in the 

 vessels may be greater or less according to cir- 

 cumstances ; and there may occur within the 

 skull local determinations or partial distribu- 

 tions of the blood. When from rupture of a 

 bloodvessel, inflammation, suppuration, or other 

 causes, blood, serum, or pus are effused into 



* Edin. Mcd. Chirurg. Trans, vol. i. 



the cavity of the cranium, the circulating blood 

 must be diminished in quantity ; when there is 

 any obstruction to the return of the blood by 

 the jugular veins, the pressure of the blood en- 

 tering by the carotid artery is proportionally 

 greater; and when the arteries which supply 

 blood to the brain are obstructed, or the heart's 

 action is less forcible than usual, the pressure 

 on the brain must be diminished in a corre- 

 sponding degree. 



In the natural state of the circulation the 

 pressure exerted by the blood circulating 

 through the cranium is subject to regular alter- 

 nations of increase and decrease from the effect 

 of the heart's action and the motions of respira- 

 tion. When the brain of man or of animals is 

 exposed by the removal of a part of the skull, it 

 is seen to be slightly raised at the exposed part 

 at each arterial pulsation, and more perceptibly 

 during each expiration. The brain falls again 

 during each succeeding inspiration, but does 

 not sink below the level of the skull. These 

 motions may also be perceived at the fontanelles 

 of the infant's head, where the bony parietes of 

 the skull are deficient. In the closed state of 

 the skull, for the reasons previously mentioned, 

 it is obvious that there can be no motions simi- 

 lar to those observed in the brain when ex- 

 posed, but nevertheless the brain must be more 

 forcibly pressed upon by the blood at these 

 times than at others. Haller, who had observed 

 these motions, conceived the depression during 

 inspiration to be caused simply by the ease 

 with which the blood enters the chest at that 

 time, and attributed the swelling of the brain 

 during expiration to the obstacle then offered 

 to the descent of the blood through the jugu- 

 lar veins. It seems, however, probable that 

 the greater fulness of the arteries during 

 expiration may also contribute to raise the 

 brain at the time when the collapse of the 

 walls of the chest occurs : for Magendie ob- 

 served, that when a ligature was put upon the 

 jugular vein, the blood which issued from this 

 vein by an aperture above the ligature, flowed 

 with greater force during expiration, shewing 

 that increased arterial pressure during expira- 

 tion was continued through the capillaries into 

 the veins. Sign. Ravina, who made a very 

 extensive series of experiments upon these mo- 

 tions, found that when the brain has been de- 

 pressed during inspiration, it again swells, 

 although no expiration succeeds, but that when 

 raised during expiration, it does not again sink, 

 if inspiration does not follow. 



3. Influence of varieties in the distribution 

 of arteries and veins upon the circulation. As 

 connected with some of the above-mentioned 

 facts, and exerting a considerable influence in 

 modifying the circulation of the blood in parti- 

 cular states of the animal economy, we may 

 here mention a few of the more remarkable 

 varieties in the distribution of the arteries and 

 veins, together with the uses they have been 

 supposed to serve in different animals. The 

 varieties of form in the larger arteries may be 

 considered under two heads; a, simple tor- 

 tuosity ; and b, sudden division into many 

 small branches. 



