532 OF SECRETION. 



tion, and the plan on which the Hepatic circulation is carried on. The 

 secretion of the Liver is formed from blood conveyed to it by one large vessel, 

 the Vena Portae, which has collected it from the Venous capillaries of the 

 chylopoietic viscera, and which subdivides again to distribute it through the 

 liver. The secretion of the Kidney, in like manner, is elaborated from blood 

 which has already passed through one set of capillary vessels, 'those of the 

 Malpighian tufts ; this blood is collected and conveyed to the proper secreting 

 surface, not by one large trunk (which would have been a very inconvenient 

 arrangement), but by a multitude of small ones, the efferent vessels of the 

 Matpighian bodies, which may be regarded as collectively representing the 

 Vena Portae, since 4hey convey the blood from the systemic to the secreting 

 capillaries. Hence The Kidney may be said to have a portal system within 

 itself. This ingenious view of Mr. Bowman's finds support from the fact that 

 in Reptiles (in which, as in Fishes, the Portal trunk receives the blood from 

 the whole posterior part of the body, and supplies the Kidneys as well as the 

 Liver), the efferent vessels of the Malpighian bodies which receive .their 

 blood, as elsewhere, from the Renal Artery unite with the branches of the 

 Portal vein, to form the secreting plexus around the Tubuli Uriniferi. Here, 

 therefore, the blood of the secreting gle-xus has a double source ; the vessels 

 which supply it receiving their blood irr part from the capillaries of the organ 

 itself, and in part from those of viscera external to it ; just as, in the Liver, the 

 secreting plexus is supplied in part bijl the blood conveyed from the chylo- 

 poietic viscera through the Vena Portae, and in part by the nutritive capillaries 

 of the organ itself, which receive their blood from the Hepatic Artery. 



670. The nature and purpose of the Urinary secretion, and the alterations 

 which it is liable to undergo in various conditions of the system, are much 

 better understood than are those of the Bile; this is owing, in great part, to 

 the circumstance that it may be readily collected in a state of purity ; and 

 that its ingredients are of such a^^ure, as to be easily and definitely sepa- 

 rated from each other by simplecllRfcic'al means. There can be no doubt that 

 the chief purpose of this excretidn is to remove from the system the effete 

 azotized matters, which the bloo^ takes up in the course of the circulation, or 

 which may have been produced by changes occurring in itself. This is evi- 

 dent from the large proportion of Nitrogen which is contained in the solid 

 matter dissolved in it ; and from the crystalline form presented by this solid 

 matter when separated a form which indicates that its state of combination 

 is such as to prevent it from conducing to the nutrition of the system. The 

 injurious effects of the retention in the blood of the%omponents of the Urinary 

 secretion are fully demonstrated by the results of its cessation ; whether this 

 be made to take place experimentally (as by tying the renal artery), or be the 

 consequence of a disordered condition of the kidney. Symptoms of great dis- 

 order of the nervous centres, analogous to those produced by many narcotic 

 poisons, soon exhibit themselves ; and the patient dies comatose, if the secre- 

 tion be not restored. In such cases, Urea (the characteristic ingredient of the 

 urine) is found to have accumulated in the Blood ; and it may even be delected 

 by the smell, in the fluid effused into the Ventricles of the Brain. The 

 conclusion which may be drawn from this circumstance, regarding the pre- 

 existence in the Blood of the components of the secretion, is strengthened by 

 the fact that, even in the healthy state, Urea may be detected in the blood ; 

 it only exists there normally, however, in very small quantity ; but when 

 there is any impediment to its excretion, it goes on accumulating, and produces 

 consequences more or less serious in proportion to its amount. It is not im- 

 probable that, as in the case of the retention of Bile in the Blood ( 661), 

 many of the minor as well as of the severer forms of sympathetic disturbance, 

 connected with disordered secretion from the Kidney, are due to the directly 



