508 THE BLOOD-VASCULAR SYSTEM 



than in the great arteries and veins. From the relative slowness of the blood flow in the sys- 

 temic capillaries, it has been estimated that their total bed is eight hundred times greater than 

 the bed of the main arterial stem. 



Variations in the com-se and arrangement of the adult arteries and veins, originally studied 

 by the surgeon for utihtarian purposes only, now furnish one of the most stimulating fields for 

 anatomical research. Text-books can provide, at best, catalogues of the arrangement commonly 

 found in the adult body and of the most ordinary variations. That no text-book description 

 can fit any individual case in all particulars, and that unusual distribution of vessels does not 

 necessarily shorten fife are among the earliest lessons learned in the anatomical laboratory. 

 The adult vascular pattern is derived from a symmetrical arrangement in the early embryo 

 of which scarcely a trace remains. The intervening changes are so numerous and profound that 

 the general uniformity of vascular distribution in different individuals is more remarkable 

 than the occurrence of occasional wide variations from the usual type. 



In early stages of development all vessels have a similar structure; they consist, in fact, of 

 a single layer of endothehum. Some vessels, however, are larger than others; these act as 

 arteries or veins (according to the direction of flow) while the smaller channels perform the office 

 of capillaries. The early principal vessels do not necessarily persist, for many of these dwindle 

 or are lost. New channels are meanwhile in continuous process of formation and some of these 

 may, in turn, become main channels. It thus follows that the main vessels of the adult must 

 be looked upon rather as selected channels through a plexus of possible pathways, than as sepa- 

 rate entities which must necessarily conform to given rules of distribution and branching. 



In time, no doubt, most of the commoner variations from the usual adult type will receive 

 a rational explanation; at present enough has been done to indicate the value of the embryo- 

 logical method. The list of variations in the arteries and veins respectively is preceded by a 

 brief account of the morphogenesis of these vessels. 



In the case of the heart anomalies frequently result in early death, so that subjects of devel- 

 opmental irregularities are seldom seen in the anatomical laboratory. The anomalies usually 

 consist in improper development of the septa which normally divide the heart and main ar- 

 terial trunk into their pulmonary and systemic halves. A short account of the morphogenesis 

 of the heart is appended to the description of the adult organ. 



In the following section the heart and pericardium will first be considered 

 followed by the arteries and veins. 



^ 



A. THE HEART AND PERICARDIUM 



1. THE HEART 



The heart [cor] is a hollow organ principally composed of muscle, the myo- 

 cardium. It is lined iDternally by endocardium which is continuous with the 

 intima of the blood-vessels. Externally, it is covered by the epicardium, a serous 

 membrane continuous with the serous lining of the pericardium. The form of 

 the heart, when removed from the body without previous hardening, is that of a 

 fairly regular truncated cone. The base [basis cordis] is poorly circumscribed 

 but corresponds, in a general way, to the area occupied by the roots of the great 

 vessels and the portion of the heart-wall between them. The base of the heart 

 is held in position* chiefly by the great vessels, which are attached to the peri- 

 cardium; the remainder of the organ is capable of free movement within the 

 pericardial cavity. 



The interior of the heart is longitudinally divided, into right and left cavities, 

 by a septum passing from base to apex. Each cavity is subdivided into an 

 atrium [atrium cordis] and a ventricle [ventriculus cordis], the former receiving 

 the ultimate venous trunks and the latter giving rise to the main arteries. Thus 

 the left atrium receives the four pulmonary veins, and the right atrium the 

 superior and inferior vena cava and the coronary sinus; the aorta issues from the 

 left ventricle and the pulmonary artery from the right. The ventricles, which 

 constitute the major portion of the heart, may be recognised by their very thick 

 walls. The atria have; thinner walls and arc less capacious than the ventricles; 

 projecting from (!a('h is a diverticulum or auricle [auricula cordis]. The auricles 

 (which r(!Coivo their name from their res(;ml)lance to dog's ears) partially embrace 

 the roots of tin; pulmonary artery and aorta. 



Orientation of the heart. — The apex of the heart [apex cordis] points forward, 

 to the left and downward. The base is dirticted backward, to the right and up- 

 ward. The longitudinal axis of the heart forms an angle of about 40° with the 

 horizontal plane and also with the median sagittal plane of the body. 



* Not necessarily fixed, for during systole the base performs a greater excursion than does 

 the apex. 



