OF THE VESSELS IN GENERAL. 249 



and in the plexus choroides, fringes in the pia mater, a trellis in 

 the pituitary membrane, tufts in the crystalline; they are arched 

 in the iris, &c. These modes of formation are so constant and 

 regular, that by examining with the microscope a small por- 

 tion of a well injected organ, we may easily recognise to which 

 part it belongs.* 



367. The vessels are more or less diaphanous, according 

 to their thickness. They are whitish. Whatever the density 

 of their parietes may be, especially at their inner surface, they 

 are permeable in the cadaver, and even in the living body, 

 either from without inwards, or from within outwards. Their 

 tenacity or cohesion is considerable ;t but is not the same in 

 the three kinds, in everyone of their parts, nor in the various 

 coats with which they are composed. The same is the case 

 with respect^o their elasticity,^: which is generally con- 

 siderable, ancfwhich exists either in the fibres lengthwise, or 

 in those encircling the vessels. They are evidently irritable, 

 and their vital contractility is generally in an inverse ratio to 

 their elasticity. They are not distinctly sensible. Their 

 power of formation is very active. 



368. The vessels are canals through which the circulating 

 humours continually pass and moisten every part of the body; 

 they together with the heart are the organs or agents of this 

 movement, both by their elasticity and by their organic or vital 

 contractility. 



369. The formation and development of the vascular sys- 

 tem, have been particularly observed in the chicken in its 

 shell, rather less in the foetus of the mammiferous animals, and 

 little in the human species. i 



The veins, especially those of the umbilical vesicle, are 

 formed before the heart and arteries. It is uncertain, if in the 

 allantoid or umbilical vessels, the veins are also formed before 



* See Soemmering loc cit, Prochaska he cit. 



j- Cl. Wintringham, experimental inquiry on some parts of the animal struc- 

 ture; London, 1740. 



$ D. Hoffman, Diss. inaug- med. de elastidtatis effedibus in machind hu~ 

 mana; 1734. 



