GENERAL ANATOMY. 361 



axis of the mesli running parallel with the long axis of the nerve or fibre. Some- 

 times the capillaries have a looped arrangement, a single vessel projecting from 

 the common network, and returning after forming one or more loops, as in the 

 papillae of the tongue and skin. 



The number of the capillaries, and the size of the meshes, determine the degree 

 of vascularity of a part. The closest network, and the smallest interspaces, are 

 found in the lungs and in the choroid coat of the eye. In the liver and lung, the 

 interspaces are smaller than the capillary vessels themselves. In the kidney, in 

 the conjunctiva, and in the cutis, the interspaces are from three to four times as 

 large as the capillaries which form them ; and from eight to ten times as large as 

 the capillaries of the brain in their long diameter, and from four to six times as 

 large in their transverse diameter. In the cellular coat of the arteries, the widrti 

 of the meshes is ten times that of the capillary vessels. As a general rule, the 

 more active the function of an organ is, the closer is its capillary net, and the 

 larger its supply of blood ; the network being very narrow in all growing parts, 

 in the glands, and in the mucous membranes ; wider in bones and ligaments, which 

 are comparatively inactive ; and nearly altogether absent in tendons and cartilages, 

 in which very little organic change occurs after their formation. 



Structure. The walls of the capillaries consist of a fine, transparent, homoge- 

 neous membrane, in which are imbedded, at intervals, minute oval corpuscles, 

 probably the remains of the nuclei of the cells from which the vessel was origi- 

 nally formed. The largest capillaries have a trace of an epithelial lining, and a 

 few filaments circularly dispersed. 



In the description of the arteries, we shall first consider the efferent trunk of 

 the systemic circulation, the aorta, and its branches ; and then the efferent trunk 

 of the pulmonic circulation, the pulmonary artery. 



THE AOETA. 



The aorta (iopri}, arteria magna) is the main trunk of a series of vessels, which, 

 arising from the heart, conveys the red oxygenated blood to every part of the 

 body for its nutrition. This vessel commences at the upper part of the left ven- 

 tricle, and, after ascending for a short distance, arches backwards to the left side, 

 over the root of the left lung, descends within the thorax on the left side of the 

 vertebral column, passes through the aortic opening in the Diaphragm, and enter- 

 ing the abdominal cavity terminates, considerably diminished in size, opposite 

 the fourth lumbar vertebra, where it divides into the right and left common iliac 

 arteries. Hence its subdivision into the arch of the aorta, the thoracic aorta and 

 the abdominal aorta, from the direction or position peculiar to each part. 



ARCH OF THE AORTA. 



Difsection. In order to examine the arch of the aorta, the thorax should be opened, by divid- 

 ing 1 the cartilages of the ribs on each side of the sternum, and raising this bone from below up- 

 wards, and then sawing through the sternum on a level with its articulation with the clavicle. 

 By this means, the relations of the large vessels to the upper border of the sternum and root of 

 the neck are kept in view. 



The arch of the aorta extends from the origin of the vessel at the upper part 

 of the left ventricle, to the lower border of the body of the third dorsal vertebra. 

 At its commencement, it ascends behind the sternum, obliquely upwards and 

 forwards towards the right side, and opposite the upper border of the second 

 costal cartilage of the right side, passes transversely from right to left, and from 

 before backwards to the left side of the second dorsal vertebra ; it then descends 

 iipon the left side of the body of the third dorsal vertebra, at the lower border of 

 which it becomes the thoracic aorta. Hence this portion of the vessel is divided 

 into an ascending, a transverse, and a descending portion. The artery in its course 

 describes a curve, the convexity of which is directed upwards and to the right 

 side, the concavity in the opposite direction. 



