124 LYMPHATIC SYSTEM. 



side, and thus both the chyle and lymph are mixed with the blood. 

 If, therefore, the thoracic duct be tied instantly after killing an 

 animal, not only the lacteals, but also the lymphatics in the ab- 

 domen and lower extremities become distended with their natural 

 fluids, the course of those fluids being stopped by the ligature. 



The lacteals, the lymphatics, and the thoracic duct, all agree 

 in having their coats more thin and more pellucid than those 

 of the blood-vessels. But although their coats are so thin they 

 are very strong, as we daily see on injecting them with mercury, 

 since they resist a column of that fluid, whose weight would 

 make it burst through blood-vessels, the coats of which are many 

 times thicker than those of the lymphatic system. 



The thinness of the coats prevents our dividing them from 

 one another, and thereby ascertaining their number, as we do 

 those of the blood-vessels. But as the blood-vessels have a 

 dense internal coat to prevent transudation, we have reason to 

 believe the lymphatics have the same. And as the blood-vessels 

 have a muscular coat, which assists in the circulation, 1 so may 



1 The existence of the muscular coat of the blood-vessels has been rendered pro- 

 bable by Dr. Verschuir's experiments, wherein these vessels were found to be irri- 

 table, and also by the following circumstance which I observed in dissecting an ass. 

 The arteries of this, like those of other animals, have a strong elastic coat, which 

 coat after distension contracts them again to a certain degree ; but this contraction 

 never goes so far as to shut up the cavity of the artery, and as it acts equally in the 

 dead as in the living body, large arteries are therefore always found with considerable 

 cavities. But in this ass, which I bled to death, the arteries contracted more than 

 their elastic coats were capable of doing ; for those of the kidneys were without a 

 cavity, and resembled a cord ; and that this contraction was muscular appeared upon 

 distending them again, in which case they stood open as they commonly do in dead 

 bodies. This fact will help us to explain why the arteries appear empty in dead 

 bodies; which may be owing to their muscular fibres having (before death) con- 

 tracted to the degree seen in this animal, by which means all the blood was driven 

 into the veins ; but these muscular fibres ceasing to act after death, the elastic fibres 

 overcome that contraction, and expand the arteries which therefore appear empty 

 (LXIII). Since writing the above, I have dissected a still-born child which was de- 

 fective in many parts of the body, and in particular in having no heart. In this child 

 the circulation had been carried on merely by an artery and a vein, whose coats 

 therefore probably were muscular. 



(LXIII.) Hewson takes no account here of the pressure of the at- 

 mosphere. Dr. Davy a examined the carotid artery of man in nineteen 

 cases after death, and always found it collapsed, and quite or nearly 



a Researches, Physiological and Anatomical, ii, 188. 



