250 GENERAL ANATOMY. 



the arteries. It is very probable that in the body of the foetus 

 the arteries are formed before the veins. 



The vessels are observed in the thickness of the umbilical 

 membrane, under the form of small, rounded vesicles, and se- 

 parated from each other; these vesicles augment in number 

 and unite, which produces a very loose vascular net-work. 

 Their first rudiments are in the beginning deprived of proper 

 parietes, and consist in mere passages made in the substance of 

 the membrane. This substance accumulates by degrees about 

 their circumference, and this forms their parietes. The tex- 

 ture and composition of these parietes, are only developed in 

 time. 



As to the primitive simplicity of the circulation in the fce- 

 tus, its successive complication, the formation of the heart, 

 that of the pulmonary vessels, &c. more particularly belongs 

 to special anatomy, and especially to embryology,* than to 

 general anatomy. 



The number of the vessels generally and their diameter, 

 and consequently their sum total, are, as relate to the mass of 

 the body, so much the more considerable as the animal is 

 nearer the time of its formation. The vessels, in general, 

 especially the sanguiferous, and more particularly the arteries, 

 acquire considerable density in old age. 



370. The circulating system presents little differences 

 relative to the sexes; nevertheless the vessels are rather 

 thicker and firmer in the males. There is no appreciable dif- 

 ference in the races of men. 



Individual varieties, on the contrary, are very frequent and 

 very numerous in this system; they consist particularly in 

 differences of origin, volume, number and precise situation; 

 they exist nearly in the same degree in the three species of 

 vessels. 



371. Under many circumstances,accidental,and commonly 

 very minute vessels, are formed. 



Adhesions, at first simply glutinous, become afterwards 

 vascular. The same is the case with respect to the accidental 



* Ph. Beclard, Embryologie on Essai anat. sur k foetus humain, in 4to. 

 Paris, 1821. 



