TISSUES OF THE BODY. 387 



The vessels frequently undergo, subsequently, further development, both 

 in regard to form, size, and texture. Those of the gravid uterus present 

 a peculiar periodical increase in size. Others, as, for instance, those of the 

 cornea towards the close of foetal life and after birth, suifer extensive 

 obliteration. During this process His observed the formation of stellate 

 bodies resembling ramifying pigment cells. 



Pathological neoplasis of vessels is of frequent occurrence. It was for- 

 merly supposed, however, that they were formed independently of the 

 normal vessels already present, the pathological tubes becoming subse- 

 quently united to those physiologically formed. But there can be no doubt 

 that the pathological have the same mode of origin as the normal. 



Thus, in the regeneration of a tadpole's tail we may remark (fig. 387, 

 a, &, c, d) the same long known protoplasm sprouts and threads again If 



Fig. 387. Development of capillaries in the regenerating tail of a tad- 

 pole after Arnold, a, b, c, d, sprouts and cords of protoplasm. 



we look at the same vascular region twenty-four hours later, it presents 

 the appearance figured in fig. 388. The protoplasm thread, d, has become 

 converted into a pervious capillary tube ; a, b, and c have become wide 

 protoplasm cords. 



It appears, however, that a new formation of vessels may take place in 

 another manner. According to Thiersch, if a wound be made in the 

 tongue of mammals, a number of wall-less passages are observed between 

 the arteries and veins at a certain stage of the healing process, which carry 

 the blood. Some of these lacunar passages become later on converted 

 into true vessels, the neighbourhood probably supplying vascular cells, while 

 the greater number are destroyed. Hereafter, in speaking of the spleen, 

 we shall have to consider similar lacunar blood streams as existing in it. 



Vascular tumours, known as angiomas, present a different structure. "We 

 refer the student to the handbooks of pathological anatomy. 



As to the development of the lymphatic vessels we possess very little 

 information at present. There can be but little doubt, however, that the 



