'The veins are neanished by small arteries, and are supplied like the at teries, ith nerves and absor• bents.
The Vein-: arise e.. L1 1 the ill w hit It the arteries terminate from the convolutions of glands, or from spongy cells, as in the penis, and proliabiy nl the sidcel. Within the head, the VCillS CI) COM1011 nicate with the sinuses formed within the doubling:, of the dam muter.
It has been commonly supposed that the veins with in the belly differ from those in the general system ; but Bichat,w ho has examined them w ith great attention, describes the in as differing, as to structure, only in the greater density of the cellular membrane, and less fibrous appearance, mid extensibiiity of the intermedi ate coat. The distribution of that particular set of veins that are ramified through the liver, called die Porta, will be considered sti hen we treat of that organ in the seventh chapter.
'ie veins in the fatal state, and during early child hood, are proportionally much less, compared with the arteries, than at any future period of life. As age in creases they become larger, and those next the surface become more evident, and in old people they are much dilated and very prominent, especially in the atlantal ex tremities. The embos.ved appearance of the armg of an old woman, which has furnished our poets with one of the most striking features in their portrait of a witch, is owing partly to this dilatation of the veins, and partly to the absorption of the hit contained in the cellular sub stance.
The names of the principal veins, and their corres pondence with arteries, will he noticed immediately in the general table of arteries.
Of the Capillary and E.rhalant r e88( We have seen that the most general termination of arteries and commencement (If veins, am; by e•y mi nute or capillary vessels ; and this observation is appli cable both to the circulating vessels of the general sys tem, and to those that arc confined to the lungs. Capil lary vessels also form the convolutions of secreting glands. Their offices, in all these situations, must of course be different. In some they appear merely to con tinue the circulation of the blood from the arteries into the veins, while in the lungs they are supposed to be the principal seat of that important and necessary change which the blood undergoes in circulating through these organs ; and in the glands they are doubtless subser vient to the function of secretion. With respect to
their structure and properties we know very little. The microscope informs us that they are cylindrical and trans parent, but of what membranes they are composed, and whether they possess an inherent contractibility, or merely admit the passage of the fluids urged on by the contractions of the heart and arteries, we cannot deter mine.
It is found that various matters are separated from the general mass of blood that passes through the arte ries, w ithout any very complex organ to serve as the me dium of separation. Thus, into all the cavities of the body there is poured a watery or a mucous liquor, which serves to moisten both the sides and the cavities, and the peripheral surface of their contents ; again, a moisture is (Mused through the pores of the skin, and passes through the cuticle in the limn of vapour or 01 fluid ; and a similar clrusiou takes place in the lungs. As no particular apparatus can be discovered by which the se paration is effected, it is naturally coil( hided, either that the moisture exudes through pores in tile sides of the blood-vessels, or of the membranes lining the cavities, or that it is poured out of exhaled by particuLe vessels wandering from the general course of circulation, and Opening within the cavities in the cellular part of the lungs, or below the skin. These separatrig vessels have long been known to anatomists by the name of ex halants. They are generally Considered as coming off From the arteries ; and llichat is of opinion that they proceed from the capillary vessels.
Sonic anatomists consider these vessels as of the same nature with those that supply the secretary glands, and call them all by the general name of NIT( ruing vessas ; While others contend that these exhalants exist only ill the brains of the humoral pathologists. We wish not to interfere NS ith these disputes on the ininu!Le of anatomy, and shall therefore, for the present, dismiss the subject of the exhalant arteries, with referring such of our readers as wish for a full account of them, to Bichat's ./natoinie Gem rate, tom. ii.