222 



THE CIRCULATORY SYSTEM. 



chyma of the organ," without the intervention of connective tissue; 

 or, when this is present, usually only in small quantity, it is 

 secondarily acquired, since in the early developmental stages of 

 sinusoids no connective tissue intervenes between them and the 

 parenchyma of the tissue. They develop by the intergrowth and 

 intercrescence of the parenchyma of the organ and venous endothe- 

 lium. Sinusoids are found in the following organs : liver, suprarenal, 

 heart, parathyroid, carotid gland, spleen, and hemolymph glands. 



(d] Anastomoses, Retia mirabilia, and Sinuses. In the 

 course of certain vessels, abrupt changes are seen to occur as, for 

 instance, when a small vessel suddenly breaks up into a network 

 of capillary or precapillary vessels, which, after continuing as such 

 for a short distance, again unite to form a larger blood-channel, 

 the latter then dividing as usual into true capillaries. Such struc- 

 tures are known as retia mirabilia, and occur in man in the kid- 



Sensory nerve-ending. 



Plexus of vasomotor nerves. 



r 



Fig. 177. Small artery from the oral submucosa of cat, stained in methylene- 

 blue, and showing a small portion of a sensory nerve-ending and the plexus of vasomotor 



ney, intestine, etc. Again, instead of breaking up into capillaries, 

 a vessel may empty into a large cavity lined by endothelial cells 

 (blood sinus). The latter is usually surrounded by loose con- 

 nective tissue and is capable of great distention when filled with 

 blood from an afferent vessel, or when the lumen of the efferent 

 vessel is contracted by pressure or otherwise. The cavernous or 

 erectile tissue of certain organs is due to the presence of such 

 sinuses (penis, nasal mucous membrane, etc.). If vessels of larger 

 caliber possess numerous direct communications, a vascular plexus 

 is the result ; but if such communications occur at only a few 

 points, we speak of anastomoses. Especially important are the 

 direct communications between arteries and veins without the 

 mediation of capillaries. Certain structural conditions of the tis- 

 sue appear to favor such anomalies, which occur in certain exposed 







