CHAP. VIL] THE CAT'S ORGANS OF CIRCULATION. 193 



called " lacteals," which are to be found in the intestinal villi. These 

 lacteals we shall see open into tubes called " lymphatic vessels," or 

 " lymphatics;' and which vessels ultimately open and pour their 

 contents into certain veins, after traversing here and there in their 

 course certain bodies called lymphatic glands. The lymphatics do 

 not contain " blood," but a colourless fluid called "lymph? consisting 

 in part of the nutritious material absorbed from the walls of the ali- 

 mentary canal, and partly of such of the colourless matter of the blood 

 as has exuded from the vessels in order to effect nutrition, but 

 has not been made use of. It is therefore taken up again by the 

 lymphatics to be by them reconveyed to the blood-vessels. We 

 have thus two nutritive fluids" blood " and " lymph " enclosed 

 in two systems of vessels, "blood-vessels" and "lymphatics." 



The blood-vessels form a system of tubes completely closed, save 

 at the apertures where the lymphatics open in them. In part, the 

 vessels are of microscopic dimensions, but in one place the system is 

 dilated into a large, complexly formed, rhythmically contractile 

 organ the heart. 



The heart may be considered as the central portion of the circu- 

 lating system, all the other channels being subsidiary to it. These 

 latter may be divided into three categories : (A) the vessels taking 

 blood from the heart which vessels are called arteries; (B) the 

 vessels taking blood towards the heart which are the veins ; and 

 (C) certain minute tubes which convey the blood to the tissues, and 

 intervene between and connect the ends of the arteries and veins 

 the capillaries. 



It may be well before proceeding to examine in detail these 

 various parts, and those other parts which compose the lymphatic 

 system, to consider the two fluids which these two sets of organs 

 respectively convey. 



2. The BLOOD is a thickish alkaline fluid, somewhat heavier than 

 water, which has a saltish taste and a faint odour, and is of a more 

 or less scarlet or more or less purple red, according to circumstances. 

 It consists mainly (more than 75 per cent.) of water, with a con- 

 siderable quantity (12 to 14 per cent.) of an albuminoid substance 

 termed hemoglobin, the rest consisting of albumen and other protein 

 matters and salts. A nitrogenous substance called fibrin may be 

 obtained from fresh- drawn blood by whipping it with slender rods 

 the fibrin then adhering to the rods in the form of a soft, whitish, 

 stringy matter. Though apparently homogeneous to the naked eye, 

 blood spontaneously separates (when drawn from the body and 

 allowed to stand undisturbed) into different parts one fluid, the 

 other more or less solid. 



This process of solidification is called COAGULATION,* and it occurs 

 thus : the fresh-drawn blood forms itself into a jelly-like mass, 



* Fibrin, as such, does not exist in 

 the blood. It is supposed by some to be 

 formed by the chemical union of two 



albuminous substances, which exist side 

 by side in the blood while alive. 



