380 MANUAL OF HISTOLOGY. 



nective-tissue corpuscles. Eecklinghausen does not mean, however, that a system of 

 hollow cells is formed of these, but rather a series of fine clefts as it were (Saftka- 

 nalchen) traversing the tissue, in which elementary spheroidal (connective-tissue) 

 cells are lodged. This view, however, has met with the greatest opposition, and is, 

 we are convinced, incorrect. In order to account for the transition of these "sap- 

 canals " into the larger lymphatic radicals, which are lined with cells, Eeckling- 

 hausen points to the stomata already mentioned in the text. Many of these small 

 figures, however, may possibly have another significance. They may, namely, be 

 nothing more than separated processes of some of the vascular cells, possibly designed 

 to provide for the increase in size of the vessel. Klein and Burdon- Sanderson both 

 speak of another set of openings on the serous sacs besides those of the lymphatics 

 just mentioned ; these are the terminations of the sap carialiculi on the free surface 

 of the membrane, and have been named by them " jpseudo-stomata ." (3.) In the 

 fluid of serous sacs Eecklinghausen has found lymph corpuscles, from which fact it 

 might be imagined that the formation of such lymphoid elements could take place 

 from the epithelial cells of these cavities. Ludwig and Schweigger-Seidel allow that 

 such a multiplication and transformation of the epithelium cells on the centrum 

 tendineum of the rabbit may take place. Kodliker informs us also that on the 

 human peritoneum, and especially in that part, namely, forming the great omentum, 

 there occur a number of aggregations of epithelial cells in the form of nodulated 

 excrescences filled with lymphoid cells. An explanation of this phenomenon has, 

 however, since been offered by the discovery of the migration of lymphoid cells. 



209. 



As to the physiological relations of the vessels, we will only discuss, in 

 as few words as possible, a few of their leading features. We have 

 already seen that the thicker wall of the arteries chiefly depends upon 

 the well-developed middle coat; that it displays greater richness in 

 transverse layers of unstriped muscle, interleaved with elastic plates, 

 while veins of the same calibre have thin walls, owing chiefly to thinness 

 in the tunica media, while the tunica adventitia is strongly developed. 

 "We have likewise seen that in the smaller veins the muscular elements 

 soon disappear completely, whilst in the very smallest arteries, even down 

 to their termination in the capillaries, these contractile fibre-cells are 

 distinctly visible. The capillaries themselves are entirely destitute 

 of muscular tissue, but possess, according to Strieker, vital contractility 

 ( 202). > 



The circulation of the blood takes place, as is well known, with pulsa- 

 tions through the arterial vessels, and evenly through the veins and 

 capillaries. The pressure of the blood upon the walls of the arteries is 

 very considerable, exceeding at least tenfold that upon the internal sur- 

 face of veins, and increasing, besides, more and more with the ramifications 

 of the former. 



The walls of the larger vessels, in keeping with their texture, possess a 

 slight but very perfect elasticity, that is, they can be stretched by a small 

 amount of force, returning subsequently to their original form. It must 

 be borne in mind, too, that the vascular tubes are always largely dis- 

 tended with blood, so that the elasticity of their walls also exercises a 

 certain amount of pressure on the columns of fluid enclosed by them. If 

 we consider this centred in the arteries, whose expansion under increas- 

 ing pressure is much less than that of the veins, we have in them a 

 system of elastic tubes charged with blood, into which, at every contrac- 

 tion of the heart, a new quantum of the latter is forced. The pulsation 

 of arteries is an undulation produced in the walls of the latter by this 

 pumping in of fresh quantities of fluid, and is gradually destroyed as it 

 advances toward the periphery by the resistance of the vessels, which are 

 undergoing an ever-increasing ramification. Owing to this the capillaries 



