SECOND SECTION. 

 THE ARTERIES. 



CHAPTER I. 



GENERAL CONSIDERATIONS. 



THE name of arteries is given to the centrifugal vessels, which carry 

 the blood from the heart to the various organs. These vessels proceed from 

 the heart by two trunks, which are perfectly independent in the adult 

 animal ; they originate, one in the right ventricle, the other in the left. 



The first of these trunks, destined to carry the dark blood, is the pul- 

 monary artery. The second conveys the red blood, and is named the aorta. 

 There exist, therefore, two groups of arteries ; the pulmonary system, and 

 the aortic system. 



GENERAL FORM. Single at their origin, the two arterial systems soon 

 divide into less voluminous trunks, which again subdivide into successively 

 decreasing canals, until at last their diameter becomes reduced to an extreme 

 degree of tenuity. In a word, the arterial trunks present the ramous dis- 

 position of dicotyledonous plants. The total volume of the secondary trunks 

 exceeds that of the primary trunk, and the same relation exists between the 

 respective dimensions of the branches and their ramifications, to the ultimate 

 divisions of the artery. In tracing all the ramifications of one of these systems 

 to a single canal, it will then be found that this canal is incessantly increas- 

 ing from its origin to its termination, and that it represents a hollow cone 

 whose apex corresponds to the heart. 



FORM OF THE ARTERIES. Each arterial tube affects a regularly cylin- 

 drical form, whatever its volume may be. When the diameter of these 

 vessels is measured at their origin and their termination, between two 

 collateral branches, no sensible difference is perceived. 



MODE OF ORIGIN. The arterial ramifications are detached in an angular 

 manner from the parent branches which give them origin. Sometimes the 

 angle of separation is more or less acute this is most frequently the case ; 

 sometimes it is at a right angle, and at other times it is obtuse. It will be 

 readily understood that the opening of this angle exercises a somewhat 

 marked influence on the course of the blood ; for example, the blood from 

 a principal vessel, in passing into the canal of a secondary one which springs 

 from it at an obtuse angle, must experience a notable check in its impetus, 

 because of the change in direction it has to encounter , on the contrary, the 

 rapidity of the current is not modified to any appreciable degree in those 

 vessels which separate from their trunk of origin at a very acute angle. 

 Towards the point of separation, there is always remarked, in the interior of the 

 vessel, a kind of spur whose sharp border is towards the heart, thus dividing 

 the current of blood and diminishing the resistance. This spur resembles 

 in its disposition the pier of a bridge, against which the waters are divided 

 to pass on each side. (When a short trunk divides abruptly into several 

 branches, proceeding in different directions, it is termed an axis. A very 

 peculiar feature in the division of arteries, however, and one which will be 



