140 ANATOMY FOE, JSTUESES. [CHAP. XL 



4. That the blood in the descending aorta is chiefly derived 

 from that which has already circulated in the upper extremities, 

 and, mixed with only a small quantity from the left ventricle, 

 is distributed to the lower extremities ; hence the small size and 

 imperfect development of these parts at birth. 



Development of blood-vessels and corpuscles. The blood-vessels and 

 red corpuscles are formed very early in the embryo. They are developed in 

 that portion of the primitive tissue called the mesoblast. The cells which 

 are to form the vessels become extended into processes of varying length, 

 which grow out from the cells in two or more directions. The cells become 



P 



FIG. 100. ISOLATED CAPILLARY NETWORK FORMED BY THE JUNCTION OF 

 SEVERAL HOLLOWED-OUT CELLS, AND CONTAINING COLOURED BLOOD-CORPUSCLES 

 IN A CLEAR FLUID, p, p, pointed cell-processes extending in different directions 

 for unfbn with neighbouring capillaries. 



united with one another, either directly or by the junction of their processes, 

 so that an irregular network is thus formed. Meanwhile the nuclei in the 

 cells multiply, and each nucleus surrounds itself with a small amount of 

 cell protoplasm. The corpuscles thus formed acquire a reddish colour, and 

 the protoplasmic network in which they lie becomes hollowed out into a 

 system of branched canals containing fluid, in which the nucleated coloured 

 corpuscles float. The protoplasmic walls of the vessels gradually change 

 into the flattened cells which compose the wall of the capillaries, and which 

 form the lining membrane of the arteries and veins. The remaining coats 

 of the larger vessels are developed later from other cells which apply them- 

 selves to the exterior of these tubes. 



The first white corpuscles do not appear in the vessels so early as the 

 coloured ones. They probably occur in the beginning as free cells and 

 wander in from the outside. 



The new vessels which form in the healing of wounds and in the restora- 

 tion of lost parts are produced by a process which is essentially the same as 

 above described. Blood-corpuscles, however, are not produced within them, 

 and it is still a matter of doubt as to where and how the red corpuscles 

 originate after birth. The white corpuscles are undoubtedly produced to a 

 large extent in the lymphatic nodes and other lyinphoid structures. 



