504 



THE TISSUES. 



When they enter the artery they cannot be traced beyond the outer 

 coat. Many arteries are, however, without nerves ; e. g. those of 

 the cerebral and spinal substance, of the choroid plexus and the 

 placenta, as well as many arteries of muscles, glands, and mem- 

 branes. Of the veins, it is only in the larger that a few fine nerves 

 can be demonstrated, e. g. the sinuses of the dura mater, the verte- 

 bral canal, the venae cavas, and the jugular, iliac, femoral, and he- 

 patic veins. These also are both spinal and sympathetic. Luschka 

 thinks they terminate in the inner coat a point not yet decided. 



B. The Capillaries. 



The arteries terminate in the capillaries vessels always inter- 

 vening between the former and the veins; except in case of the 

 corpora cavernosa penis, and of the placenta. (Kblliker.) 



The capillaries are everywhere composed of a single layer of 

 simple membrane with nuclei; the structural transitions of the 

 small arteries into these vessels being wholly imperceptible. (Fig. 

 337, B.) 



On minute examination, the structureless membrane is sometimes 

 transparent, and with a simple contour; at others, thicker 



A. A capillary vessel from the gray matter of the human brain ; a, wall, of simple membrane ; b, 

 nucleus of the wall ; c, red blood-corpuscles. B, c. Different appearances of small arteries and 

 veins of the human pia-mater ; a, a, simple membrane ; b, b, circular fibres ; c, c, oval nuclei of the 

 internal epithelium here about to cease ; d, d, transverse indications of the circular fibres. D. Ter- 

 minal artery from the mesentery of a rabbit. (Magnified 200 diameters.) 





