1080 GENERAL ANATOMY OJR HISTOLOGY. 



vessels by penetrating their walls and thus finding their way into the extra-vascular 

 spaces. A chemical investigation of the protoplasm of the leucocytes shows the 

 presence of nucleo-proteid and of a globulin. The occurrence of small amounts of 

 fat and glycogen may also be demonstrated. 



The Blood-platelets are discoid or irregularly shaped, colorless, refractile 

 bodies, much smaller than the red cells. Considerable discussion has arisen as to 

 their significance. In spite of the fact that they have been observed in the blood- 

 vessels during life, there is, at present, a tendency to regard them as products of 

 disintegration of the white cells, or as precipitates, possibly of nucleo-proteid, and 

 not as living elements of the blood. 



Origin of the Blood-corpuscles. In the embryo the red corpuscles are developed 

 from mesoblastic cells in the vascular area of the blastoderm. These cells unite 

 with one another to form a network, their nuclei multiply in number, and around 

 some of the nuclei an aggregation of colored protoplasm takes place. After a 

 time the network becomes hollowed out by an accumulation of fluid, and forms 

 capillary blood-vessels, and in the fluid those nuclei which are surrounded by 

 colored protoplasm float as the first red blood-cells. 1 The embryonic corpuscles 

 are thus nucleated, and, further, they have the power of amoeboid movement. 

 These cells disappear in later embryonic life, to be replaced by smaller non- 

 nucleated corpuscles, having all the characters of the adult erythrocyte, which, 

 according to Schafer, are formed within certain cells of the connective tissue. 

 Small globules of reddish coloring-matter appear in the protoplasm of these cells, 

 and these eventually becoming larger, more uniform in size and disk-shaped, float 

 in a cavity which results from the coalescence of numerous vacuoles. The cells 

 becoming more hollowed join with neighboring cells to form new blood-vessels, 

 and these become connected with previously existing vessels. In post-embryonic 

 life the important source of the red corpuscles is the red marrow in the ends of 

 the long bones and especially in the ribs and sternum. Here are found special, 

 nucleated, colored cells, termed erythroblasts, which are probably direct descend- 

 ants of the nucleated, embryonic red cells. These erythroblasts by atrophy 

 and disappearance of their nuclei (or, as some observers maintain, by their 

 extrusion) and by assumption of the biconcave form are transformed into the 

 adult red corpuscle. Of the white corpuscles of the blood, the lymphocytes are 

 derived from lymphatic tissue generally, and from the lymphatic glands especially, 

 and enter the blood by way of the lymph-stream ; the hyaline cells probably 

 develop from the lymphocytes, while the eosinophile cells are believed to originate 

 mainly in the bone-marrow and possibly also in the connective tissues. 



The Plasma or Liquor Sanguinis, the fluid portion of the blood, has a yellowish 

 tint, is alkaline in reaction, and of a specific gravity of 1.028. It contains in 

 solution about 10 per cent, of solids, of which four-fifths are proteid in nature ; 

 the remainder being salts, chiefly chlorides, phosphates, and sulphates of the 

 alkali metals ; carbohydrates, chiefly sugar ; fats and soaps, cholesterin, urea, and 

 other nitrogenous extractives. The proteids are three in number, serum albumen, 

 serum globulin, and fibrinogen. Fibrinogen is a body of the globulin class, but 

 differs from serum globulin in several respects. It is the substance from which 

 the fibrin, which plays so important a part in the clotting of the blood, is derived. 



Coagulation of the Blood. When blood is drawn from the body and allowed 

 to stand, it solidifies in the course of a very few minutes into a jelly-like mass or 

 "clot," which has the same appearance and volume as the fluid blood and, like it, 

 looks quite uniform. Soon, however, drops of a transparent yellowish fluid, the 

 "serum," begin to ooze from the surface of the mass and to collect around it. 

 Coincidently the clot begins to contract, so that in the course of about twenty-four 

 hours, having become considerably smaller and firmer than the first formed jelly- 

 like mass, it floats in a quantity of yellowish serum. The clotting of the blood is 

 due to the formation of a fine meshwork of the insoluble material, fibrin, which 



1 Recent observations tend to show that the endothelial lining of the vessels and the blood- 

 corpuscles are of hypoblastic origin. 



