BLOOD-VASCULAR SYSTEM 287 



The size of the erythrocytes varies in different vertebrates, being the largest 

 in the amphibia (Amphinma) and smallest in the vertebrates (musk deer). A 

 few measurements are giving here in microns (o.ooi mm.). Where two dimen- 

 sions are given they are the length and breadth of the oval corpuscles. Musk 

 deer, 2.5/14; man, 7.7/i; hen, 7x12/1; carp, 9x15/1; frog, 16x25/*; Necturus, 

 31x58.5/*; Amphiuma, 44.5x73/*. 



In the higher vertebrates the red corpuscles arise by division of giant cells 

 (erythroblasts) in the red bone marrow, but in the young and at times of great 

 depletion of the blood new red corpuscles may be formed in the spleen and the 

 liver. At first all nucleated, but in the mammals the nucleus is soon lost. 



The leucoc3rtes or white corpuscles (divided accordingly as they 

 occur in blood or lymph into leucocytes and lymphocytes) are very 

 variable in shape (amoeboid) and may be uni- or polynucleate. By 

 their amoeboid motions they are able to pass through the endothelial 

 walls of the capillaries and to pass among the cells of the different 

 tissues, hence they are often called wandering cells. They have the 

 power of ingesting foreign bodies which renders them of value in 

 combating pathogenic organisms; and they also aid in the absorb tion 

 of fats and peptones. 



The blood plates are very little known. Their size is less than 

 that of the red corpuscles and they rapidly degenerate when drawn 

 from the vessels. They are circular or elliptical in outline. 



THE BLOOD-VASCULAR SYSTEM 



The blood-vessels include the arteries, which carry the blood from 

 the heart to all parts of the body; the veins, which bring it back, and 

 the capillaries, minute tubes, which connect the ends of the arteries 

 and veins, for the system is closed, and there is a complete circulation. 



Since all transfer of gases and nourishment takes place through 

 the capillaries, these vessels have extremely thin walls, consisting of 

 a single layer of squamous epithelium, the so-called intima. Usually, 

 as the name implies, the capillaries are very small in diameter, but 

 attention has recently been called to the sinusoids, vessels with 

 similar walls but larger in diameter, which are noticeable in some 

 developing organs, especially the liver. Here also must be mentioned 

 the retia mirabilia, places where an artery or vein suddenly breaks 

 up into a network of small vessels (often capillary) which unite 

 again, as in the chorioid plexuses of the brain, the red bodies of the 

 swim bladder, the pseudobranchs and in the glomeruli of the kid- 

 ney, to form a vessel as large as before. In the lymph nodes there 

 are similar networks of the lymph-vessels. 



