248 THE VITAL FUXCTIOiNS. 



be performed, although we are here unable to follow the 

 connexion between the means and the end. In some glands, 

 for example, the minute arteries, on their arrival at the or- 

 gan, suddenly divide into a great number of smaller branch- 

 es, like the fibres of a camel-hair pencil: this is called the 

 pencillaled structure. Sometimes the minute branches, in- 

 stead of proceeding parallel to each other after their divi- 

 sion, separate like rays from a centre, presenting a stel- 

 lated, or star-like arrangement. In the greater number of 

 instances, the smaller arteries take a tortuous course, and 

 are sometimes coiled into spirals, but generally the convo- 

 lutions are too intricate to admit of being unravelled. It is 

 only by the aid of the microscope that these minute and 

 delicate structures can be rendered visible; but the fallacy, 

 to which all observations requiring the application of high 

 inagnifying powers are liable, is a serious obstacle to the ad- 

 vancement of our knowledge in this department of phy- 

 siology. Almost the only result, therefore, which can be 

 collected from these laborious researches in microscopic ana- 

 tomy, is that nature has employed a great diversity of means 

 for the accomplishment of secretion; but we still remain in 

 ignorance as to the kind of adaptation, which must assuredly 

 exist, of each structure to its respective object, and as to the 

 nice adjustment of chemical affinities which has been pro- 

 vided in order to accomplish the intended effects."* Elec- 



* The only instance in which we can perceive a correspondence between 

 the chemical properties of the secretion, and the kind of blood from which 

 it is prepared, is in the liver, which, imlike all the other glands, has venous, 

 instead of arterial blood, sent to it for that purpose. The veins, which re- 

 turn the blood that has circulated through the stomach, and other abdominal 

 viscera, are collected into a large trunk, called the vena portas, which enters 

 the liver, and is there again subdivided and ramified, as if it were an artery: 

 its minuter branches here unite with those of the hepatic artery, and ramify 

 through the minute lobules which compose the substance of the liver. After 

 the bile is secreted, and carried off by hepatic ducts, the remaining blood is 

 conducted, by means of minute hepatic veins, which occupy the centres of 

 each lobule, into larger and larger trunks, till they all unite in the vena cava, 

 going directly to the heart. (See Kiernan's Paper on the Anatomy and Phy- 

 siology of the Liver, Phil. Trans, for 1833, p. 711.) A similar system of ve- 



