244 CIRCULATION OF THE BLOOD. 



that it is perfectly fresh. A very slight admixture, however, 

 of kept serum is fatal to the experiment. After a time, de- 

 cliue of tissue life manifests itself by a change in the appear- 

 ance of the preparation, the elements losing their plumpness 

 and distinctness of outline. Along with this change, the ves- 

 sels, and particularly the arteries, become relaxed, and the 

 normal exchange between the liquid inside and that outside 

 of the vessels is perverted, the latter increasing in such a way 

 as to render the whole animal oadematous. 



If. while the circulation is still normal, an injury is inflicted 

 on a part of the web as, for example, by applying mustard 

 to a spot on its surface it is seen that in the injured part 

 changes occur suddenly which are analogous to those which, 

 as tissue death approaches, affect the whole body. These 

 changes are known by the term .s/fl.s?'s, and form part of the 

 process of inflammation a word which is used as a general 

 expression for the local effects of injuring living parts to such 

 a degree as not to destroy their vitality at once. They are 

 best studied when scrum which contains a few corpuscles, or 

 defibrinated blood diluted with saline solution, is employed. 

 It is then seen that in any part of the web to which a so-called 

 irritant is applied, as, e. g., mustard the blood stream is re- 

 tarded, and the corpuscles crowd together in the dilated ves- 

 sels. This is not due to any property of mutual attraction 

 peculiar to the corpuscles, for the same thing happens if milk, 

 diluted with saline solution, is substituted for blood ; so that, 

 whatever be the nature of the change, its seat is not in the 

 circulating liquid itself, but in the vessels or surrounding tis- 

 sues. 



SECTION IV. FUNCTIONS OP VASOMOTOR NERVES. 



In the proceeding section the arteries have been regarded 

 merely as passive elastic tubes, dilating or contracting accord- 

 ing to the pressure exercised upon them by the circulating 

 blood. They must now be studied as not only elastic but 

 contractile. 



The arteries owe their contractility to the unstriped muscu- 

 lar fibres which they contain. These fibres shorten under the 

 influence of impressions conveyed to them by the vascular 

 nerves, which nerves, together with the automatic centre from 

 which they radiate, constitute the vasomotor nervous system. 

 Of the centre which governs arterial contraction, nothing is 

 known anatomically ; for there is no point or tract in the brain 

 or spinal cord to which vascular nerves can be traced back. 

 All that is known has been learnt exclusively by experiment. 



That there is a vasamotor centre, and that it is intracranial, 

 we learn by observing, first, that if the medulla is divided ini- 



