180 THE BLOOD AND ITS CIRCULATION. 



insects or of venomous plants (the nettle), produce by this 

 mechanism the rapid swelling by which they are distin- 

 guished. Beside the influences of changes of pressure, we 

 must also take into account the physiological properties of 

 the globules in the vicinity of these vessels, for we know 

 already, and shall soon see more particularly (study of the 

 epithelial or mucous and glandular surfaces), that there are 

 certain tissues formed of globules which act as barriers to the 

 passage of fluids while others more especially assist; in 

 other words, the tissues near the capillaries exercise more or 

 less attraction to the contents of these tissues. 



Beside these general functions, the circulatory system 

 exhibits special arrangements in certain parts, indicating 

 some special and accessary purpose ; thus the vessels, in 

 some organs, have to perform the part of supplying heat as 

 well as nutrition, as the vessels of the external ear, of the 

 face in general, -the extremities of the fingers, and the integ- 

 uments of the articulating regions ; these vessels are much 

 more numerous in all these parts than the simple purpose of 

 nutrition requires. In other parts the capillaries are arranged 

 with a special view to absorption or exhalation, as those of 

 the lung, which form in this viscus a large work of blood- 

 vessels in which the red globules become impregnated with 

 oxygen, while the serum evolves its carbonic acid. 



The afflux of the blood has also a mechanical part to play, 

 that of erection, for instance; it is in this case only, that we 

 find those accessary peripheral hearts, intended to increase 

 the tension of the blood in the organs which are capable of 

 erection : by their rhythmical contraction during erection, 

 the bulbo-cavernous and the ischio-cavernous muscles drive to 

 the extremity of .the penis the blood which has flowed into 

 the bulb of the urethra, and the root of the cavernous bodies. 



The movement of the circulation is indispensable in order 

 to keep the blood in its physiological condition, a fluid state ; 

 not that the motion prevents the coagulation of the blood ; 

 on the contrary, it promotes it, and it is by beating that the 



motor fibres. But Ranvier had satisfied himself, by previous ex- 

 periments in tying the inferior vena cava, that destruction of the 

 sensitive roots and of the voluntary motor roots at their issue from 

 the spinal cord was followed by no cedematous phenomenon in the 

 abdominal region. Paralysis of the vaso-motor nerves appears thus 

 to be the cause of the dropsy which takes possession of the limb 

 which has undergone section of the sciatic nerve. 



