244 GENERAL ANATOMY. 



why surgeons prefer making their incisions in the longitudi- 

 nal direction, in order to avoid wounding them. 



In many places, however, vessels have a fluctuating course. 

 The fluctuation of the vessel consists in a deviation or alter- 

 nate undulation of the vessels from a straight lino; it increases 

 when the vessels are turgid, or when a subject is injected, and 

 in the arteries during the systole of the heart; it diminishes 

 under opposite circumstances, and especially by closely dis- 

 secting the vessels. In the vessels of the parts subject to great 

 change of volume, figure and situation, this fluctuation is very 

 well marked; as for instance, the mouth, stomach, intestine, 

 bladder, uterus, the tongue, and testicles before they descend 

 from the abdomen,, and those immediately surrounding the ar- 

 ticulations: in these latter, however, there is less fluctuation, 

 but the vessels are very elastic. 



The vessels of the spleen and brain, and the spermatic veins, 

 form also a great many windings, although they appear not to 

 be intended for the same use. 



The fluctuations of the sanguineous vessels are more strong- 

 ly marked than those of the lymphatic vessels, and those of 

 the arteries more than those of the veins. 



358. The symmetrical distribution of the vessels is very 

 imperfect. This does not exist in the central parts; they are 

 very nearly symmetrical as to their divisions, which belong 

 to symmetrical parts, and have no symmetry in those be- 

 longing to parts which are not symmetrical. The arteries, 

 the veins and lymphatic vessels present equally this disposi- 

 tion. In certain animals and in the embryo, the vascular 

 system is more symmetrical than in the adult man. More- 

 over, besides the general want of symmetry, the vascular 

 system is also subject to many irregularities in its distribu- 

 tion. 



359. The parietes of the vessels adhere by their external 

 surface to the mass of the body in which they are ramified; 

 their internal surface is smooth, polished, humid, and in con- 

 tact with the circulating fluids; it presents a projection where- 

 ever the branches form acute angles with the trunks. The 

 thickness of the parietes, when compared with the relative 



