TISSUES OF THE BODY. 381 



are not reached by it. This undulating motion of the artery does not 

 constitute the propulsive force of the circulation; its only effect is to 

 accelerate the course of the stream of blood. The advance of the latter is 

 occasioned rather by the difference of pressure prevailing in arteries and 

 veins, each contraction of the heart forcing a new mass of blood into the 

 distended arterial tube, and each diastole abstracting a certain quantity 

 from the venous trunks, and receiving it into the auricle. 



The course of the circulation is in general very rapid, the time con- 

 sumed in the completion of its circuit amounting on an average to about 

 half & minute. The rapidity is greatest in the arteries, equalling in the 

 carotid of the horse 400 millimetres in the second, while in the veins it 

 is considerably less, being only 250 mm. in the second, as seen in the 

 jugular of the same animal. In the following section we shall see that 

 the flow of blood through the capillaries is very slow, the length of the 

 latter being also very considerable. This sluggishness depends probably 

 upon the narrowness of the arterial canals, compared with the much 

 greater extent of surface in the capillaries, and the consequent increase of 

 friction to the column of blood. The subsequent decrease in the super- 

 ficial extent of the bed of the stream, consequent on the confluence of the 

 capillaries to form veins, explains the comparative acceleration which 

 again becomes evident in the latter in the flow of the blood, which still 

 remains, however, as has been already remarked, far more tardy than 

 that in the arteries. 



The question now arises, What have (together with the elastic materials) 

 the muscular elements of vessels to say to the motion of the streams in 

 the vascular system ? 



We know that the arterial walls, which are rich in these, contract con- 

 siderably under electric and mechanical irritation, as well as under the 

 action of cold and many chemical reagents. Consequently, we cannot 

 deny the presence of vital contractility in the arteries, and, from their 

 allied structure, in the veins also. The general opinion is, that the muscu- 

 lar tissue of the vascular system is perpetually in a state during life of 

 slight tonic contraction, which supports the elastic action of the remain- 

 ing elements, entering into the composition of the wall. In that here, 

 also, as everywhere else, the action of the muscles is under the influence 

 of the nervous system, we must expect to find certain vessels narrowed 

 by the increased contraction of their muscles, and expanded on relaxation 

 of the latter. The regulating action, then, of vascular muscles on the 

 amount of blood contained in certain parts cannot be denied. Experi- 

 mental nervous physiology has shown, besides, that section of the nerves 

 supplying the vascular system gives rise to expansion of the arteries 

 (Bernard and others). We have to thank this observer for our acquaint- 

 ance with the striking fact that irritation of the sympathetic nerves 

 supplying the arteries of the submaxillary gland causes decrease in their 

 calibre, so that the blood passing through the organ is found to be dark, 

 while only a small amount of viscid saliva is secreted. Stimulation, on 

 the other hand, of the cranial nerve which enters the gland, namely, the 

 chorda tympani, produces a completely opposite effect, bright red blood 

 streaming rapidly through the part, and a copious watery secretion being 

 poured out. Other organs also, such as the parotid, kidney, and stomach, 

 manifest the same antagonism in the actions of expanding and contract- 

 ing nerves. In them, likewise, we observe that the enlarged vascular 

 territory contains during the process of secretion bright red blood. 



