THE BLOOD AND THE LYMPH. 339 



§ 223. 



Physiological remarks. — The development of the blood-vessels 

 takes place, essentially on the same type, in the heart, arteries, 

 and veins. The rudiments of all these vessels, and even of 

 the heart, are solid tracts of cells of greater or less thickness, 

 which, by the liquefaction of their interior substance, and the 

 metamorphosis of the central cells into blood- globules, become 

 cavities, which shortly coalesce, and constitute a continuous 

 passage for the blood. The heart and vessels having remained 

 for some time in this condition of cellular tubes, in which 

 state the former, moreover, exhibits contractions, the cells 

 composing the walls, with the exception of the innermost, begin 

 to elongate into fibres, and to represent the divers fibrous tissues 

 and tunics. At the same time the vessels become thicker and 

 increase in circumference, which at first is still to be referred 

 to an increase in the number of the cells, but subsequently, is 

 brought about chiefly, or even solely, by the growth of their 

 elements in length and thickness. In the fifth month of foetal 

 life, all the larger and medium-sized vessels are formed, with 

 their tunics and tissues, and it is impossible to perceive any 

 vestige of formative cells. The tissues, however, appear to be 

 still far from completion, the muscular fibres being short and 

 delicate, and instead of the strong elastic fibrous networks, we 

 perceive only finer and the finest fibrils, and in the place of 

 the elastic membranes themselves, only layers of more or less 

 coalescent, fusiform cells. The internal longitudinal fibrous 

 membrane alone, in many vessels, is at this time demonstrable 

 as a homogeneous elastic tunic immediately under the epithe- 

 lium, but in the smaller vessels this is wanting, and is replaced 

 by a layer of elongated cells, out of which it appears to be 

 developed. It is thought that similar cells are occasionally to 

 be seen also in the adult, in which the elastic inner membrane 

 is likewise merged. The muscular fibres of the heart arise, as 

 in other situations, from the union of cells, but I have not yet 

 seen how their anastomoses are formed, whether from a branch- 

 ing of certain formative cells, or by the lateral apposition of 

 small rows of cells — probably in both ways. 



The mode in which the development of the capillaries is 

 effected, differs in toto from that observed in the larger vessels. 



