FUNDAMENTAL PRINCIPLES OF DISEASE. 29 



When the membrane serves for secreting or excreting purposes, as 

 in the salivary glands or the kidneys, it is usually simple ; when it 

 serves the mechanical purpose of protecting a part, as over the tongue 

 or skin, it is invariably multiple and stratified, the surface wearing 

 away while new cells replace it from beneath. 



(7) In nerves, stellate cells are connected by their rays to each 

 other, or to fibers which conduct the nerve impressions, or they act 

 as receptacles, storehouses, and transmitters for them, as the switch- 

 board of a telephone system serves to connect the various wires. 



All these tissues are supplied with blood in greater or less quan- 

 tity. The vascularity depends upon the function which the tissue is 

 called upon to perform. If this is great, as in the tongue, the lungs, 

 or the sensitive part of the hoof, a large quantity of blood is required ; 

 if the labor is a passive one, as in cartilage, the membrane over the 

 withers, or the tendons of the legs, the vessels only reach the periph- 

 ery, and nutrition is furnished by imbibition of the fluids brought to 

 their surface by the blood vessels. 



Blood is brought to the tissues by arterioles, or the small termina- 

 tions of the arteries, and is carried off from them by the veinlets, or 

 the commencement of the veins. Between these two systems are 

 small, delicate networks of vessels called capillaries, which subdivide 

 into a veritable lacework so as to reach the neighborhood of every 

 element. 



In health the blood passes through these capillaries with a regular 

 current, the red cells or corpuscles floating rapidly in the fluid in the 

 center of the channel, while the white or ameboid cells are attracted 

 to the walls of the vessels and move very slowly. The supply of 

 blood is regulated by the condition of repose or activity of the tissue, 

 and under normal conditions the outflow exactly compensates the sup- 

 ply. The caliber of the blood vessels, and consequently the quantity 

 of blood which they carry, is governed by nerves of the sympathetic 

 system in a healthy body with unerring regularity, but in a diseased 

 organ the flow may cease or be greatly augmented. In health a tissue 

 or organ receives its proper quantity of blood; the nutritive elements 

 are extracted for the support of the tissue and for the product, which 

 the function of the organ forms. The force required in the achieve- 

 ment of this is furnished by combustion of the hydrocarbons and 

 oxygen brought by the arterial blood, then by the veins this same 

 fluid passes off, less its oxygen, loaded with the waste products, which 

 are the result of the worn-out and disintegrated tissues, and of those 

 which have undergone combustion. The foregoing brief outline indi- 

 cates the process of nutrition of the tissues. 



Hypernutrition, or excessive nutrition of a tissue, may be normal 

 or morbid. If the latter, the tissue becomes congested or inflamed. 



