102 ANATOMY. 



Semilunar Valves, guard the aortic orifice, and are larger and stronger than 



those on the right side. 

 Sinus Aortici, or sinus of Valsalva, a pouch between each valve and the 



beginning of the aorta. 

 Columns Carnece,e are smaller and more numerous than on the right side ; 



the Musculi Papillares are but two in number. 



Describe the Structure of the Heart. The muscular fibres take origin 

 from the four fibrous rings at the auriculo-ventricular and aortic openings. 

 The fibres of the auricles are arranged in two layers, a superficial and a deep 

 one, the latter having looped fibres and annular fibres. In the ventricles the 

 fibres are superficial and deep, the latter being arranged circularly, the former 

 spirally, coiling inwards at the apex of the heart into a whorl-like form, the 

 vortex. 



THE ARTERIES. 



What are the Arteries ? Cylindrical vessels which carry arterial blood to 

 the body from the heart. Those going to the lungs with the returning veins, 

 form the Lesser or Pulmonic Circulation. The aorta with its branches and the 

 returning veins, constitute the Greater or Systemic Circulation. The arteries 

 anastomose or communicate freely with each other everywhere throughout the 

 body, permitting the establishment of collateral circulations. 



Describe the Structure of the Arteries. They are dense, very elastic, 

 preserving their cylindrical form, and are composed of 3 coats, an Internal or 

 serous; a Middle, which is of muscular and elastic tissue; and an External, 

 of connective tissue. They are generally included in a fibro-areolar invest- 

 ment, the Sheath, which also encloses the accompanying veins. The larger 

 arteries are nourished by the Vasa Vasorum, blood-vessels which ramify in the 

 external and middle coats; and are supplied with nerves, the Vaso-motor, 

 derived from both the sympathetic and cerebro-spinal systems, and forming 

 intricate plexuses on the larger trunks. 



What are the Capillaries ? Minute vessels forming a network throughout 

 the tissues of the body between the terminating arteries and the commencing 

 veins. Their average diameter is about the -j^W ^ an mcn > an< ^ their walls 

 consist of a transparent homogeneous membrane continuous with the innermost 

 layer of the arterial and venous walls. 



[In the following pages, main branches are in italics, sub-branches in Roman.] 



Describe the Aorta. It is the main trunk of the systemic arteries, com- 

 mencing at the aortic opening of the left ventricle of the heart, arching back- 

 wards- over the roof of the left lung into the thorax, where it descends on the 

 left of the spinal column, and after passing through the aortic opening in the 

 diaphragm, it terminates in the right and left common iliac arteries opposite 



