THE CIRCULATION OF THE BLOOD 



and by the solar, celiac, and mesenteric ganglia. The vascular nerves for 

 the different organs may be given in tabulated form: 



Vascular Nerves for the Abdominal Viscera. 



Organ. Spinal Origin of the Vascular Nerves. Course to the Organ. 



Stomach and in- i T _ n yT (Splanchnic nerves and 



testine. [ 5 ' ' 7 ' ' 9 ' IO ' ' ' 3 ' '( solar and celiac ganglia. 



( Splanchnic nerves and 

 Spleen 3, 4, 5, 6, 7, 8, 9, 10, n, 12, 13 D, i L j golar an d celiac ganglia. 



j Splanchnic nerves and 



Liver 3> 4, 5, 6, 7, 8, 9, i> " D \ so lar and celiac ganglia. 



; 4, 5, 6, 7, 8, 9, 10, n, 12, 130, i, 2, 3, { Splanchnic and celiac 



Kidnev \ 4 L \ ganglia. 



< Inferior splanchnic and in- 

 Pelvic viscera. . . . i, 2, 3, 4~L -j ferfor mesenteric ganglia. 



The Vascular Nerves for the External Genital Organs. The vaso- 

 dilators for these organs arise from the second and third sacral nerves and 

 pass to the organs by the nervi erigentes and the pelvic plexus. They form 

 the second great exception to the region of general outflow of vascular nerves. 

 The constrictors, on the other hand, arise in the spinal nerves from the last 

 dorsal and first four lumbar. They run the same course as given in the table 

 for the pelvic viscera. 



The greatest variations in the quantity of blood contained at different 

 times in the external genital organs are found in certain structures which 

 contain what is known as erectile tissue. These organs, under ordinary cir- 

 cumstances, are soft and flaccid, but at certain times they receive an un- 

 usually large quantity of blood, become distended and swollen by it, and 

 pass into the state termed erection. Such structures are the corpora cavernosa 

 and corpus spongiosum of the penis of the male, and the clitoris in the female. 

 The nipple of the mammary gland in both sexes, and, according to some 

 authors, certain nasal membranes contain erectile tissue. 



The corpus cavernosum of the penis, which is the best example of an 

 erectile structure, has an external fibrous membrane or sheath. From the 

 inner surface of the sheath numerous fine lamellae project into the cavity, 

 dividing it into small compartments, like cells when they are inflated. Within 

 these cells there is a plexus of veins upon which the erectile property of the 

 organ mainly depends. The plexus consists of short veins with very close 

 interfacings and anastomoses with very elastic walls admitting of great varia- 

 tions in size. They collapse in the passive state of the organ, but are capable 

 of an amount of dilatation which exceeds beyond comparison that of the arteries 

 and veins which convey the blood to and from them. The strong fibrous 

 tissue lying in the intervals of the venous plexuses, and the external fibrous 

 membrane or sheath with which it is connected, limit the distention of the 

 vessels and give to the organ its condition of tension and firmness. The 

 same general condition of vessels exists*in the corpus spongiosum urethrae, 



