158 MANUAL OF HISTOLOGY. 



brachial, and subcutaneous branches there is a media of con- 

 siderable dimensions. The inferior vena cava has, in addition 

 to a transverse layer of muscle-cells, a longitudinal one of 

 greater thickness, and, besides these, contains muscle-cells, 

 which are scattered through its elastic coat. The veins of the 

 meninges of the encephalon and cord, the retina, the bones, 

 and the muscles, and the jugular, the subclavian, the innomi- 

 nate, and the thoracic portion of the vena cava are all entirely 

 devoid of a true muscular coat. The veins of the gravid ute- 

 rus have only longitudinal muscle-elements. In addition to an 

 outer longitudinal layer, the vena cava, the azygos, the renal, 

 the hepatic, the internal spermatic, and the axillary veins pos- 

 sess an inner circular layer. The iliac, the femoral, the popli- 

 teal, and several other veins contain a middle coat of transverse 

 muscle-cells, between internal and external longitudinal layers. 



The valves of the veins consist of longitudinal bundles of 

 connective tissue commingled with scanty elastic fibrils, and 

 containing nucleated cells. The inner endothelial layer appears 

 to be a direct continuation of the intima of the vein. That 

 portion of the subendothelial tissue which does not face the 

 blood-current is less developed than the part turned toward it ; 

 the elastic fibres of the latter are also barely visible. The at- 

 tached valvular border frequently presents transversely dis- 

 posed muscle-elements. Eberth has denied their occurrence, 

 but they have been repeatedly observed by Banvier and other 

 competent histologists. 



Peculiar vascular structures. The following structures are 

 remarkable for the conspicuous and characteristic development 

 of their blood-vessels the vascular membranes, tunicw vascu- 

 losce, such as the pia mater of the brain and spinal cord, and 

 the choroid coat of the eye. In these we find that the excessive 

 vascularity is intended to nourish, not the membranes them- 

 selves, but the organs which they invest. 



Blood-vascular glands, vascular plexuses. In man, two 

 bodies of peculiar structure represent this group. They are 

 the coccygeal gland of Luschka, and a rudimentary organ 

 called the inlercarotid gland. Both consist essentially of con- 

 voluted blood-vessels and nerves, imbedded in a nucleated con- 

 nective-tissue stroma. The coccygeal gland is a small, rounded, 

 pinkish body, of rather firm consistence, and is connected by a 

 pedicle with the middle sacral artery. This pedicle contains 



