The branch with the median nerve enlarged to a " median " artery, has been already men tioned ; it passes under the annular ligament as it enters the hand, and may reinforce the deficient radial or ulnar ; bnt most frequently the latter of the two, by joining the super ficial palmar arch.
Finally, as to the varieties in the hand, the mode in which a diminished superficial palmar arch is obviated, has already been described; and an unusually small deep arch is compen sated by the ulnar communicating, which is generally- little inferior in size to the radial contribution. For individually smaller digital branches are substituted enlarged dorsal meta carpal ; and in the case of the magna pollicis the superficial palmar arch, the superficialis volm, or the median artery, may either of them make up the deficiency.
It may be desirable to attempt a generaliza tion of these special variations; in order to this, let us return for a moment to the ordinary anatomy of the vessels of the forearm and hand considering them as a whole. Such a view assisted somewhat, it must be confessed, by our knowledge of these varieties, would dis cover in the forearm five longitudinal trunks, all possessing some feature, whether of size, length, or constancy, which especially recom mends them to our notice. They are the radial, ulnar, and anterior interosseous vessels, together with the posterior branch of this lat ter, and the branch with the median nerve. The anastomosis and distribution of the ex tremities of most of the,se, forms around the wrist an arterial circle which is much more pronounced posteriorly.* In the hand, two arches which are continuations of the larger vessels occupy its surface of flexion, at diffe rent heights and depths ; defended from the pressure inseparable from prehension by a strong fascia, whose protective effect is aided during flexion by a tightening muscle. They join by aoastomosis with the extremities of the longitudinal vessels, or the imperfect anterior carpal arch. Three branches run lengthwise in most of the metacarpal interspaces ; one on the dorsum from the posterior part of the carpal circlet, two at different depths in the palm from these arches; the dorsal and deeper palmar uniting at the superior extremities of those intervals, and all three inosculating at their inferior terminations near the clefts of the fingers.
All the varieties above tnentioned would be referrible to the increased development either of one of these longitudinal branches, or of some portion of this complete and large anas tomosis. The several varieties are, in fact, an exaggeration by turns of a different vessel ; which in its course towards distribution may return its contents to the ordinary channel by any one of these series of inosculations ; whe ther it be the superficial or deep arch, the posterior carpal arch, or finally, the superior or inferior extremity of the aforesaid interos secrus spaces.
Thus from these vessels alone might be de duced the law, of which the origin of the ob turator from the epigastric, or the sublingual from the facial, are familiar and important in stances; viz. that varieties of arteries occur as the exaggerations of an ordinary anasto mosis t : while it is no less evident that the deviations are compensative in the strictest sense ; e. that the amount of blood entering the limb is no ways affected, for that an in crease of one is a diminution of some other vessel — or vice versti.
The directness of these inosculations, and the frequency- of these resulting irregularities together exert an important influence on sur gical practice, which may be regarded in three points of view. Firstly, it necessitates unusual care in the ordinary operations ; since we may open an artery of dangerous size, where we least expect it. Secondly, it renders opera tions undertaken on the vessels themselves liable to immediate non-success ; • for we may find only a twig where we expect an artery of influential magnitude. Thirdly, it may also cause their mediate failure ; the width and number of the anastomosing chan nels rendering deligation of a trunk useless, by filling it in a very short space of time below the ligature. Fortunately, however, the same position that renders them more liable to in jury affbrds somewhat of a substitute for the operation by also e'xposing them more directly to external pressure.