8o ELEMENTS OF HISTOLOGY. [Chap. x. 



retina, and of the membranes of the brain and cord. 

 Those of the gravid uterus have only longitudinal 

 muscular fibres. The vena cava, azygos, hepatica, 

 spermatica interna, renalis and axillaris, possess an 

 inner circular and an outer longitudinal coat. The vena 

 iliaca, cruralis, poplitea, mesenterica, and umbilicalis 

 possess an inner and outer longitudinal and a middle 

 circular muscular coat. The intima of the venae 

 pulmonales in man is connective tissue containing 

 circular bundles of non-striped muscular cells (Stieda). 



104. The trunk of the vense pulmonales possesses 

 striped muscular fibres, these being continuations of 

 the muscular tissue of the left auricle. 



105. Hoyer showed that a direct communication 

 exists between arteries and veins without the inter- 

 vention of capillaries as in the matrix of the nail, 

 in the tip of the nose and tail of some mammals, in 

 the tip of the fingers and toes of man, in the margin 

 of the ear lobe of dog and cat and rabbit. 



In the cavernous tissue of the genital organs 

 veins make large irregular sinuses, the wall of which 

 is formed by fibrous and non-striped muscular tissue. 



106. (D) The capillary blood-vessels are 

 minute tubes of about -^oV o" to -3 oVo ^ an ^ ncn i n 

 diameter. Their wall is a single layer of transparent 

 elongated endothelial plates, separated by thin lines of 

 cement substance (Fig. 48) ; each cell has an oval 

 nucleus. In fact, the wall of the capillaries is merely 

 a continuation of the endothelial membrane lining 

 the arteries and veins. 



In some places the capillaries possess a special 

 adventitia made up of branched nucleated connective 

 tissue cells (hyaloidea of frog, choroidea of mammals), 

 or of an endothelial membrane (pia mater of brain 

 and cord, retina and serous membranes), or of adenoid 

 reticulum (lymphatic glands, His). 



The smallest capillaries are found in the central 



