THE BLOOD-VESSELS. 151 



nuclei. The protoplasm of these central nuclei rapidly becomes 

 liquefied, thus forming the blood-plasma. And now we have a 

 system of communicating tubules, containing corpuscles float- 

 ing in a plasma, their walls consisting of nucleated cells. 

 Hence, the blood-vessels do not arise as intercellular spaces, 

 but are hollowed out to form channels in an originally solid 

 reticulum of protoplasm derived from mesoblastic cells. 



This explanation of the way in which vessels are formed 

 aids us in understanding both how capillaries are reproduced 

 in the adult, and their regeneration under pathological condi- 

 tions. The capillary-wall itself, under the influence of favor- 

 ing circumstances, begins to bud, as it were ; the delicate proto- 

 plasmic sprouts send out more delicate filaments, which, uniting 

 with similar offshoots from neighboring vessels, establish a 

 connection between two capillaries. In due time these solid 

 structures undergo the familiar process of hollowing out, and 

 the newly formed vessel is complete. Frequently the proto- 

 plasmic threads communicate, forming a reticulum which Ran- 

 vier has called nasoformative network. This author also ob- 

 served that capillaries develop from special cells, termed 

 vasoformative cells. They resemble leucocytes, and form by 

 their prolongations a network of solid protoplasm. This is 

 originally quite independent of already existing capillaries. 

 Subsequently, however, a consolidation is effected, and the 

 blood then flows through these new channels in the usual 

 manner. 



The author has been able to trace collections of emigrated 

 leucocytes through various stages of progressive development, 

 culminating in the formation of true capillaries. The experi- 

 mental investigations on this subject were carried out in Pro- 

 fessor v. Rindfleisch's laboratory, and have been fully described 

 by his former assistant, Dr. Ziegler, of Wurzburg. 



The arteries. If we follow the capillaries in a direction 

 toward the heart, we soon find the endothelial tube receiving 

 an investment of unstriped muscle-cells. These are wound 

 transversely or obliquely around the capillary, thus forming a 

 second tube, as it were, surrounding the first. External to the 

 muscular layer there appears some connective tissue, mingling 

 with which elastic elements may be observed. The direction 

 of these additional fibres is mainly longitudinal. They form 

 the third or external coat, called the adventilia, the second or 



