CIRCULATION. 



669 



this not the case, the absence of the heart might 

 be attended in these malformed productions 

 with an unusual development of muscular 

 power in parts of the vascular system.* 



In conclusion, we may remark that the argu- 

 ment drawn from the occurrence of circula- 

 tion apparently little impaired through arteries 

 which have been completely ossified for a con- 

 siderable time, seems to be very much in favour 

 of the view we have taken that the heart alone 

 is the cause of the progressive flow of blood 

 through the arterial tubes. 



3. Phenomena of the capillary circulation. 

 The phenomena of the passage of the blood 

 from the terminations of the arteries into the 

 commencement of the veins through the capil- 

 lary vessels, are highly interesting and impor- 

 tant in many points of view, for the immediate 

 respiratory change which the venous blood 

 undergoes in the pulmonary vessels, and all 

 those alterations of composition which accom- 

 pany nutrition, growth, secretion, and other 

 organic processes connected with the systemic 

 vessels, occur in the smallest ramifications of 

 the pulmonic and systemic circulation, and the 

 morbid state of inflammation as well as the 

 various pathological changes which occur as its 

 consequences are intimately connected with an 

 altered condition of the capillary system. 



a. Structure and distribution of the capillary 

 vessels. The name of capillary is generally 

 given to all those minute vessels which form 

 the means of communication between the 

 small ramifications of the arteries and veins ; 

 but there is some difference in the opinion of 

 anatomists and physiologists as to how much 

 of the vascular system ought to be included 

 under the division of the capillary vessels. 

 Some, adhering to the strict meaning of the 

 term, apply it to all the small vessels whatso- 

 ever under a certain size; others hold that 

 between the extremities of the arteries and 

 veins there is always situated a series of minute 

 tubes of nearly equal size in their whole length, 

 and not ramifying like the arteries or veins, 

 which constitute a system of vessels distinct 

 from the others in their structure, distribution, 

 and properties, to which the name of capillary 

 ought to be restricted .f The last view appears 

 to us to be founded in a partial acquaintance 

 with the system of minute vessels, for though 

 it may be true that in some parts of animals 

 the capillaries have obviously the structure 

 above described, and seem to form a system of 

 vessels apart from the smaller arteries and 

 veins, yet this is by no means the case in other 

 textures ; and we think that the more extensive 

 observation of the structure of these vessels in 

 various parts will shew that in the greater 

 number, as is well ascertained to exist in 

 many, the smaller arteries pass into veins 

 quite in a gradual manner, the ramifications 

 of each class of vessel becoming more and 



* See the Researches of Elben, Tiedemann, 

 Breschet, and others on Acephalous Monsters. 



t Dr. Marshall Hall's Essay on the Circulation 

 of the Blood, Lond. 1831. Dr. James Black's 

 Short Inquiry into the Capillary Circulation, Lond. 



more minute until they meet, the two kinds 

 of vessel presenting no difference of character 

 other than the change of direction assumed 

 by the moving blood, which enables us to 

 say with certainty where the artery termi- 

 nates, and at what point the vein begins, 

 and affording thus no reason to consider the 

 continuous tube by which they join as different 

 in structure from either the minute artery or 

 vein. While we acknowledge therefore the 

 importance of the observations which point 

 out the existence of capillary vessels of a uni- 

 form size in some textures, we think it necessary 

 to retain the name of capillary as applied to all 

 the minute vessels, both for the reason that the 

 communicating vessels are not every where of 

 the same kind, and that from the use already 

 made of the term by physiological writers its 

 meaning will thus be more easily understood. 



The vessels which lead from arteries to veins 

 are of very various sizes, some admitting only 

 one globule at once, others being so large as to 

 allow of the passage of three, four, or even a 

 greater number of red globules together. In 

 tracing with the microscope the motion of the 

 minute streams of blood as they pass through 

 the capillary vessels, the eye is guided by the 



Fig. 330. 



Frog's foot. 



