92 CIRCULATION OF THE BLOOD. 



carefully-conducted observations, in which the author, by means of various irritants, 

 succeeded in producing very curious and interesting phenomena, which he regarded as 

 inflammatory. It is not our object to discuss the nature of inflammation or to treat of the 

 changes in the character of the capillary circulation which are supposed to attend this 

 condition, as this subject is entirely pathological ; but it must be remembered, in con- 

 sidering the effects of direct irritation on the capillary circulation, that the phenomena 

 thus observed in cold-blooded animals cannot be taken as absolutely representing the 

 characters of inflammation in the human subject. When an irritation is applied to a 

 transparent part, the phenomena observed may be due to many causes, as the direct 

 effects upon the contractile elements of the blood-vessels, reflex action through the 

 nervous system, and the direct influence of the application upon the constitution of the 

 blood. Saline or other fluids are competent to modify, to a very considerable extent, the 

 composition of the blood, when separated from it only by the thin, permeable walls of the 

 vessels; and the phenomena which follow their application are necessarily very complex. 

 The process of inflammation is by no means completely understood ; but it is pretty gen- 

 erally acknowledged to be a modification of nutrition. We are hardly prepared to admit 

 that this modification, whatever it may be, can be induced under our very eyes, simply 

 by the application of irritants. With these views, microscopical researches on the " state 

 of the blood and blood-vessels in inflammation" do not assume the importance which is 

 attributed to them by many authors. Keeping this in mind, we may state the following 

 as a summary of the phenomena which have been observed in the capillary circulation, 

 as the result of irritation applied to transparent parts : 



The application of the irritant is immediately followed by constriction of the arterioles 

 and diminution in the rapidity of the current in them as well as in the capillaries. 



This constriction of the vessels is but momentary, if a powerful irritant, like a very 

 strong solution of a salt, be used. It is followed by a dilatation of the vessels and an 

 increase in the rapidity of the circulation. 



Soon after the vessels have become dilated, the rapidity of the circulation becomes 

 progressively diminished, until oscillation of the blood in the vessels takes place, which 

 occurs when the circulation is about to cease. This oscillation finally gives place to com- 

 plete stagnation ; and the vessels become crowded with blood, so that the transparent 

 layer next their walls is no longer observed. In this condition, it has often been noticed 

 that the proportion of colorless corpuscles is increased. 



Following the contraction and subsequent dilatation of the vessels, there are stasis and 

 engorgement of the parts which have been exposed to irritation. If the irritation bo 

 discontinued, this condition is gradually relieved, and the blood resumes its normal 

 current. 



In inflammation, as it is observed in the conjunctiva and in other vascular parts, there 

 unquestionably is congestion of the vessels ; but there is no positive evidence of stagnation 

 of blood in the parts as a constant occurrence. The circulation seems, indeed, to be more 

 active than in health. With regard to the microscopical phenomena just mentioned, the 

 contraction of the arterioles is simply the effect of a stimulus upon their muscular coats; 

 and dilatation takes place probably in consequence of the excessive contraction, for it has 

 been shown that this condition of the muscular fibres is pretty constantly followed by 

 unusual relaxation. It has never yet been determined how far the stasis of the blood is 

 due to an osmotic action of solutions employed in the experiments. 



Circulation of the Blood in the Veins. The blood, distributed to the capillaries of all 

 the tissues and organs by the arteries, is collected from these parts in the veins and 

 carried back to the heart. In studying the anatomy of the capillary system or in ob- 

 serving the passage of the blood from the capillaries to larger vessels in parts of the 

 living organism which can be submitted to microscopical examination, it is seen that the 

 capillaries, vessels of nearly uniform diameter and anastomosing in every direction, give 



