132 



HAND-BOOK OF PHYSIOLOGY. 



ihemperivascular lymphatic sheaths (Fig. 121). Lymph channels are 

 said to be present siso in the tunica media. 



Nervi Vasorum. Most of the arteries are surrounded by a plexus 

 of sympathetic nerves, which twine around the vessel very much like ivy 

 round a tree: and ganglia are found at frequent intervals. The smallest 



f ' Iff 



FIG. 113. Blood-vessels from mesocolon of rabbit, a. Artery, with two branches, showing tr. n. 

 nuclei of transverse muscular fibres ; I. n. nuclei of endothelial lining; t. a. tunica advent! tia. v. 

 Vein. Here the transverse nuclei are more oval than those of the artery. The vein receives a small 

 branch at the lower end of the drawing: it is distinguished from the artery among other things by its 

 straighter course and larger calibre, c. Capillary, showing nuclei of endothelial cells. X 300. 

 (Schofield.) 



arteries and capillaries are also surrounded by a very delicate network of 

 similar nerve-fibres, many of which appear to end in the nuclei of the 

 transverse muscular fibres (Fig. 122). It is through these plexuses that 

 the calibre of the vessels is regulated by the nervous system (p. 152). 



THE CAPILLARIES. 



Distribution. In all vascular textures, except some parts of the 

 corpora cavernosa of the penis, and of the uterine placenta, and of the 

 spleen, the transmission of the blood from the minute branches of the 

 arteries to the minute veins is effected through a network of microscopic 

 vessels, called capillaries. These may be seen in all minutely injected 

 preparations; and during life, in any transparent vascular parts, such 

 as the web of the frog's foot, the tail or external branchiae of the tadpole, 

 or the wing of the bat. 



The branches of the minute arteries form repeated anastomoses with 



