156 



The flow of the blood into the arteries which are distributed to mus- 

 cles, is not interrupted, when these muscles contract, for, whenever arte- 

 ries, of a certain calibre, penetrate into muscles, they are surrounded by 

 a tendinous ring 1 , which, during the contraction of the muscle, becomes 

 enlarged, from the extension in every direction, effected by the fibres 

 which are attached to it, around its circumference. The existence of 

 this truly admirable conformation, may be readily ascertained, by observ- 

 ing the aorta, in its passage through the crura of the diaphragm; the 

 perforating arteries of the thigh, where they enter at the back part of the 

 limb into the adductor muscles, the popliteal, as it passes through the 

 upper extremity of the soleus muscle. 



LX. Of the capillary -vessels. The arteries, after dividing into branches, 

 these branches into lesser ones, and these into progressively smaller 

 ramifications, terminate in the tissue of our organs, by becoming conti- 

 nuous with the veins. The venous system arises, therefore, from the ar- 

 terial system, the origin of the veins being merely the more minute ex- 

 tremities of the arteries, which becoming capillary from the great number 

 of divisions* they have undergone, bend in an opposite direction, and 

 become altered in their structure. 



These minute capillary arteries form with the minute veins, with which 

 they are continuous, and with the lymphatics, wonderful meshes in the 

 tissue of our organs. 



Several physiologists consider the capillary blood-vessels as an inter- 

 mediate system between the arteries and veins, in which the blood, en- 

 tirely out of the influence of the action of the heart, flows slowly, with an 

 oscillatory and sometimes retrograde motion, is no longer red, because 

 its globules are strained, as it were, and, in a manner, lost in a colourless 

 serum, which serves them as a vehicle. 



It is, in fact, necessary, that bodies should be of a certain bulk, to re- 

 flect the rays of light at an angle sufficiently obtuse, that the eye may dis- 

 cover their colour. We know, that grains of saiid reduced to a very fine 

 dust, appear colourless, when examined separately, and are seen to pos- 

 sess colour, only when in a state of aggregation : further, very thin lami- 

 na of a horny substance, appear transparent, though the part from which 

 they have been detached be of a red or blue colour. But if several of 

 these transparent lamina be laid on one another, the red colour becomes 

 darker, in proportion as a greater number are brought together. 



Let irritation, from whatever cause, determine the blood to flow into 

 the serous capillary vessels, in greater quantity, and with more force, 

 these vessels will become apparent, the "organs in whose structure they 

 circulate, will acquire a red colour, more or less deep; thus the conjunc- 

 tiva, the pleura, the peritoneum, the cartilages, the ligaments, &c. which 

 naturally, are whitish or transparent, become red, when affected with 

 inflammation, whether from the increased impetus of the circulation, 

 which forces and accumulates into the capillary vessels, a greater num- 

 ber of red globules, or that the sensibility of these small vessels is im- 



* The arterial divisions which may be discerned by the aid of anatomy, do not ex- 

 ceed eighteen or twenty : nevertheless, they divide still further, when they are become 

 so minute us not to be discernible without the help of the most powerful microscope,' 



Note. 



