THE MOTION OF THE BLOOD. 163 



creeping upon it c ; it gives very great tone and elasticity to the 

 arteries. 



" II. The middle coat consists of transverse fibres d , lunated 

 or falciform, and almost of a fleshy nature : hence this has the 

 name of muscular coat, and appears to be the chief seat of the 

 vital powers of the arteries. 



" III. The inner coat lining the cavity of the arteries is highly 

 polished and smooth," and is called the serous coat. It is brittle, 

 so as to be cracked by a blow, a ligature fixed around the 

 whole artery, or torsion of the vessel, while the external coat 

 remains entire. The middle coat may give way at the same time, 

 but frequently lacerates, through the pressure of the blood, by 

 degrees only ; and the external coat will remain entire, merely 

 dilated into a pouch, for a length of time, a state called 

 false aneurysm. Dr. Hales found the carotid of a dog burst at 

 once by the pressure of a column of water less than 190 feet 

 high. 6 



" This is much more distinct in the trunks and larger branches 

 than in the smaller vessels. 



" Every artery originates, either 



" From the pulmonary artery (the vena arteriosa of the 

 ancients), which proceeds from the anterior ventricle of the 

 heart, and goes to the lungs ; 



" Or from the aorta, which proceeds from the posterior ven- 

 tricle, and is distributed throughout the rest of the system. 



" These trunks divide into branches, and these again into 

 twigs, &c. 



" According to the commonly received opinion, the united 

 capacity of the branches, in any part of the sanguiferous system, 

 is greater than that of the trunk from which they arise. But I 

 fear that this is too general an assertion, and even that the 

 measure of the diameter has been sometimes improperly con- 

 founded with that of the area. I myself have never been able 

 to verify it, although my experiments have been frequently 

 repeated, and made, not on vessels injected with wax, after the 



e *' Fr. Ruysch, Respons. ad ep. problematicam. iii. Also his Thesaur, Anat. iv. 

 tab. 3." 



d B. S. Albinus, Annot. Academ. Uiv. tab. 5. fig. 1." 

 e Hamastatics. 



M 4 



