47 MATERIA MEDIC A AND THERAPEUTICS. 



flammation, characterised by capillary dilatation and escape of 

 the constituents of the blood, which can be induced by a con- 

 tinuation of the same measures, or by excessive heat, Cantha- 

 rides, Croton Oil, etc. (vesicants and pustulants), and markedly 

 modifies, as we shall see in chapter xiv., the capillary circulation 

 of neighbouring parts, and the general blood pressure. 



On the other hand, we can contract the capillaries and 

 diminish the flow through them by the application of excessive 

 local cold (congelation and refrigeration), by Lead, and Silver, 

 which are pure astringents ; and by the constringents, namely, 

 Tannic and Gallic Acids, and the many vegetables which contain 

 them (Kino, Catechu, etc.), which constringe or "tan" the con- 

 nective tissues supporting the delicate capillaries, by condensing 

 their gelatinous and albuminous constituents. Some substances, 

 such as Persalts of Iron, may also arrest the circulation in the 

 capillaries, by promoting coagulation of the blood within them. 



5. Our influence upon the walls of the veins appears to be 

 but small. The veins of a part may be dilated by hot applica- 

 tions ; contracted, and then dilated, by moderate local cold. 

 Ergot is believed by some authorities to relax the venous walls. 

 Indirect measures are more powerful in our hands. The heart 

 afronte, or the arterial pressure a tergo, may be employed, as 

 we have seen, to increase or diminish the venous pressure. The 

 processes of secretion and excretion are not less powerful in 

 modifying the fulness of the veins. Thus, hydragogue purga- 

 tives, as we have seen, drain the portal system ; and we shall 

 afterwards find that saline diuretics relieve the renal veins in a 

 very similar way. 



III. PATHOLOGICAL RELATIONS. 



The complex circulatory apparatus is subject to many forms 

 of derangement and disease, a few only of which require to be 

 noticed for the purpose of illustrating the application of drugs 

 and other therapeutical measures. 



1. Disorders of the heart and vessels belong chiefly to three 

 classes, according to their causes : (a) They may be due to 

 direct nervous causes, such as mental excitement or depression, 

 or to some cause acting reflexly through 'the nervous centres in 

 the medulla, such as derangement of the stomach, intestines, 

 uterus, etc. (b) They may originate in morbid states of the 

 blood, especially anaemia, which disturbs the centres in the 

 medulla, the vessels, and the nervo-muscular structures in the 

 heart. Or (c) they may be traced to a poison in the system, e.g. 

 tobacco, tea, alcohol, lead, and the poison of gout, each of which 

 has a specific action on some part of the mechanism. 



