PECULIARITIES OF CIRCULATION. 387 



known, especially in certain diseased states ; and the keeping an inflamed part 

 at as high a level as possible, is often one of the most important therapeutic 

 means that can be adopted. 



V. Peculiarities of the Circulation in different parts. 



517. In several portions of the Human body, there are certain varieties in 

 the distribution, and in the functional actions of the Blood-vessels, which should 

 not be omitted in a general account of the Circulation. Of these, we have in 

 the first place to notice the apparatus for the Pulmonary circulation ; the chief 

 peculiarity of which is, that venous blood is sent from the heart, through a 

 tube which is Arterial in its structure, whilst arterial blood is returned to the 

 heart, through a vessel whose entire character is that of a Vein. The move- 

 ment of the blood through these is considerably affected by the physical state 

 of the Lungs themselves ; being retarded by any causes which can occasion 

 pressure on the vessels (such as over-distension of the cells with air, ob- 

 struction of their cavity by solid or fluid depositions, or by foreign substances 

 injected into them, &c.) ; and proceeding with the greatest energy and regu- 

 larity, when the respiratory movements are freely performed. The Portal 

 circulation, again, is peculiar, in being a kind of offset from the general or 

 systemic circulation ; and also in being destitute of valves; and it may be sur- 

 mised with much probability, that the purpose of their absence is, to allow of 

 an unusually free passage of blood from one part of that system to another, 

 during the very varying conditions to which it is subjected ( 709). 



518. Another very important modification of the circulating system, is that 

 which presents itself within the Cranium. From the circumstance of the 

 cranium being a closed cavity, which must be always filled with the same total 

 amount of contents, the flow of blood through its vessels is attended with some 

 peculiarities. The pressure of the atmosphere is here exerted, rather to keep 

 the blood in the head than to force it out ; and it might accordingly be inferred 

 that, whilst the quantity of cerebral matter remains the same, the amount of 

 blood in the cranial vessels must also be invariable. This inference appears 

 to derive support from the experiments of Dr. Kellie.* On bleeding animals 

 to death, he found that, whilst the remainder of the body was completely ex- 

 sanguine, the usual quantity of blood remained in the arteries and veins of the 

 cranium ; but that, if an opening was made in the skull, these vessels were 

 then as completely emptied as the rest. It is not to be hence inferred, how- 

 ever, that the absolute quantity of blood within the cranium is not subject to 

 variation ; and that in the states of inflammation, congestion, or other morbid 

 affections, there is only a disturbance of the usual balance of the arterial and 

 venous circulation. The fact in all probability is rather that the softness of 

 the cerebral tissue, and its varying functional activity, render it peculiarly liable 

 to undergo alterations in bulk ; and that the amount of the cerebro-spinal fluid 

 varies considerably at different times ; so that the quantity of blood may thus, 

 even in a healtfiy condition, be continually changing. Moreover, in disordered 

 states of the circulation, the quantity of blood in the vessels of the cranium may 

 be for a time diminished by sudden extravasation, either of blood or serum, 

 into the cerebral substance ; and the amount of interior pressure upon the walls 

 of the vesssels may also be considerably altered, even when there is no differ- 

 ence in the quantity of fluid contained in them.t 



519. The Erectile tissues constitute another curious modification of 

 ordinary vascular apparatus. The chief of these are the Corpora Cavernoss 



* Edinburgh Medico-Chirurgical Transactions, vol. i. 



+ The results of the more recent experiments of Dr. G. Burrows (Med. Gaz., April 

 May, 1843) fully confirm the views stated above. 



