266 THE BLOODVESSELS, BY C. J. EBERTH. 



Besides the above-named elements the vascular walls contain 

 elastic fibres and sheets, which sometimes appear as finer or 

 coarser fibres arranged in a retiform manner, at others in the 

 form of strong broad bands, and sometimes as fine striated 

 lamellae and membranes. The elastic fibres form a network 

 extending through the whole thickness of the investing layer, 

 the proportional development of which varies not only in 

 different portions of the vascular system, but also in the different 

 coats. Such fibrous networks attain a great development in 

 the arteries on the external surface of the muscular tunic, where 

 they often form a strong and tolerably well-defined layer. 

 (Henle's external elastic coat.) 



YASA VASORUM AND NERVES. The tunica ad ventitia of the 

 large arteries and veins possesses arteries, capillaries, and veins 

 which may extend even into the external layers of the muscular 

 coat. The inner fibrous membrane is destitute of vessels. 



Lymphatics have not hitherto been traced into the coats of 

 the bloodvessels. The lymphatics of the endocardium only 

 extend as far as the semi-lunar valves * 



In Amphibia and Reptiles, the large vessels, and especially 

 the arteries, lie in the interior of immense lymphatic spaces, 

 and are invested by the cell membrane of the lymphatics. 



The perivascular spaces in the brain and spinal cord of 

 Mammals, which were formerly regarded by His as lymphatics,*)* 

 are, according to his more recent investigations, as well as mine, 

 only lacunae in the tissue, and possess no proper walls. 



With the exception of the capillaries, the presence of nerves 

 has been demonstrated in all vessels, even in the tunica adven- 

 titia of the non-muscular veins of the pia mater. These, partly 

 consisting of dark-edged and partly of pale fibres, break up 

 after they have traversed the tunica adventitia into a fine net- 

 work. The fibres of this network, according to my observations 

 on the small cutaneous vessels of the frog, are of the most deli- 

 cate description, whilst the network is of the closest character. 



* Eberth and Belajeff, Virchow's Archiv, Band xxxvi., 1866, p. 124. 

 t Zeitschrift fur wissenschaftliche Zoologie, Band xv., 1865, p. 127. 



