AORTA. 263 



partially encircle the cylinder of the tube in an oblique direction. 

 Upon the surface the cellular tissue is loose, to permit of the move- 

 ments of the artery in distension and contraction. 



The middle or fibrous coat is composed of yellowish fibres of 

 elastic tissue, which are disposed in an oblique direction around the 

 cylinder of the vessel, and cross each other in their course. This 

 coat is elastic and fragile, and thicker than the external coat. Its 

 elasticity enables the vessel to accommodate itself to the quantity of 

 blood which it may contain ; its fragility is exhibited in some cases 

 of aneurism, and in the division of the two internal coats of an artery 

 by a ligature. 



The internal coat is a thin serous membrane which lines the in- 

 terior of the artery, and gives it the smooth polish which that sur- 

 face presents. It is continuous with the lining membrane of the 

 heart, and through the medium of the capillaries with the venous 

 system. The internal is connected to the fibrous coat by a close 

 cellular tissue which is very liable to disease and depositions of 

 various kinds ; and is the seat of the first changes which precede 

 aneurism. The researches of Henle have demonstrated an epithe- 

 lium, composed of vesicles and scales, with central nuclei, upon the 

 surface of this internal coat, analogous to the epithelium of serous 

 and mucous membranes. 



The arteries in their distribution through the body are included 

 in a loose cellular investment which separates them from the sur- 

 rounding tissues, and is called a sheath. Around the principal ves- 

 sels the sheath is an important structure ; it is composed of cellulo- 

 fibrous tissue, intermingled with tendinous fibres, amd is continuous 

 with the fasciae of the region in which the arteries are situated, as 

 with the thoracic and cervical fascia? in the neck, transversalis and 

 iliac fasciae, and fascia lata in the thigh, &c. The sheath of the 

 arteries contains also their accompanying veins, and sometimes a 

 nerve. 



The coats of arteries are supplied with blood like other organs of 

 the body, and the vessels which are distributed to them are named 

 Vasa vasorum. They are also provided with nerves ; but the mode 

 of distribution of the nerves is at present undiscovered. 



In the consideration of the arteries, we shall first describe the 

 aorta, and the branches of that trunk, with their subdivisions, which 

 together constitute the efferent portion of the systemic circulation : 

 and then the pulmonary artery as the efferent trunk of the pulmo- 

 nary circulation. 



AORTA. 



The Aorta arises from the left ventricle, at the middle of the root 

 of the heart, opposite the articulation of the fourth costal cartilage 

 with the sternum. At its commencement it presents three dilata- 

 tions, called the sinus aortici, which correspond with the semilunar 

 valves. It ascends at first to the right, then curves backwards and 



