32 Introduction to Animal Morphology. 



a closed system of vessels appears, larger vessels 

 traversing the tissues and carrying pure, nutritious 

 blood (arteries), ending in fine capillary vessels, in 

 which this blood acts as a pabulum to the tissues, 

 finally ending in larger vessels (veins), which return 

 the blood impoverished by the loss of some of its 

 nutritious parts, and carrying away the products of 

 tissue-waste. In Vertebrates, this system is still more 

 perfect and better organized. The venous blood re- 

 quires to be purified and to receive fresh nutritive 

 elements. The first end is accomplished by its being 

 exposed to oxygen, also by its passing in contact with 

 certain gland cells which abstract its impurities.* 



* Blood is a connective tissue with a fluid intercellular matrix (called 

 liquor sanguinis) and corpuscular cells. The simplest form of cells are 

 amoeboid, sometimes vacuolated or simple, protoplasm : in the higher Verte- 

 brates these co-exist with reddish discoidal corpuscles. These corpuscles 

 have a firm central basis (zooid) composed of the nucleus and haemoglobin 

 inhabiting the cecoid or disc. 



The red corpuscles consist of paraglobulin and haemoglobin, a crj'stal- 

 lizable principle which can absorb oxygen and exist in two states, 

 deoxidised (purple cruorin) or oxidised (scarlet cruorin). 



The fluid in which the corpuscles float (hquor sanguinis) contains albumen, 

 fibrinogen, fatty matters (serolin, cholesterin, and ordinary fat, phosphates, 

 chlorides, &c.) When blood is exposed to the air, at rest, or under other 

 conditions, the paraglobulin (fibrinoplastin) of the liquor sanguinis and 

 corpuscles unites with the fibrinogen of the plasma, and forms fibrin, which 

 separates from solution, entangling the corpuscles and forming the clot. 

 The fluid part (liquor sanguinis minus fibrinogen) is called the serum. 



Pure blood in the arteries or in veins returning from the depuratory 

 glands is bright scarlet containing oxidised haemoglobin, a large quantity of 

 dissolved oxygen, and a small amount of carbonic acid. In the capillaries 

 this blood nourishes the tissues, parts with some of its protoplasm-forming 

 elements and with much of its oxygen, while it takes up urea, inosite, 

 kreatin, and a larger quantity of carbonic acid. Hence venous blood is dark 

 purple (coloured by deoxidised haemoglobin), contains 6'7 per cent, more 

 carbonic acid, and 9 per cent, less oxygen than the arterial {Sczelkow). 



