318 SOLID PARTS OF ANIMALS. 



Carbon, . 53-571 



Hydrogen, . 7-026 



Azote, . 15-360 



Oxygen, . 24-043 



100.000 

 He represents its constitution by the empirical formula, 



C 48 H 38 Az 6 O 16 . If from this formula 

 we abstract C 48 H 36 Az 6 O 14 , the formula for pro- 



tein, there will remain H 2 O 2 , or two atoms of wa- 



ter. So that the middle coat of arteries may be represented by 

 1 atom protein + 2 atoms water. 



IL The veins, like the arteries, are composed of three coats ; 

 but they are much thinner and more flaccid than the correspond- 

 ing arterial coats. They are easily distended, admitting of con- 

 siderable enlargement in the transverse direction. They are al- 

 so susceptible of elongation, but not to the same extent as the 

 arteries. 



The external venous coat, like that of the arteries, consists of 

 cellular tissue, but is much thinner and less firm than that of the 

 arteries. It is very closely united to the middle coat. 



The internal coat is a thin shining membrane continuous with 

 that which lines the auricles of the heart. It is here and there 

 thrown into folds which constitute valves. It is considered by 

 anatomists as similar in its nature to the serous membranes ; 

 though I am not aware of any experiment to elucidate the point. 



The middle coat of the veins is thinner and much more pliant 

 than that of the arteries. It appears at first sight smooth and 

 even in its texture and destitute of fibres. But a more careful 

 inspection shows that it consists of fibres, chiefly longitudinal ; 

 though some few have a transverse direction. 



This middle coat has nothing in common with the middle coat 

 of the arteries. It is not elastic, and the fibres of which it is 

 composed are muscular, at least where the vena cava approaches 

 the heart 



