OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 769 



although a perfect closure may be effected by purely mechanical means in the 

 dead vein, it is more than probable that in the living one, the contraction of the 

 coats of the vessel exercises a regulating influence. 



Structure of the Arteries and Arterial Valves. 



The coats of the arteries, as is well known, are thicker than those of the veins, 

 while the layers composing them are more numerous. The external coat, accord- 

 ing to Henle, consists of an outer layer of areolar tissue in which the fibres run 

 obliquely or diagonally round the vessel, and an internal stratum of elastic tissue ; 

 the middle coat in the largest arteries, according to Rauschel, being divisible into 

 upwards of forty layers. The layers of the middle coat consist of; pale, soft, 

 flattened fibres, with an admixture of elastic tissue, the fibres and elastic tissue 

 being disposed circularly round the vessel. The internal coat is composed of one 

 or more 1 lyers of fibres, so delicate that they constitute a transparent film, the 

 film being perforated at intervals, and lined with epithelium. The arteries, as 

 might be t expected from their structure, and as was proved by the admirable ex- 

 periments of John Hunter, whose beautiful preparations I have had an oppor- 

 tunity of examining, possess a high degree of elasticity and vital contractility, 

 and are extensible and retractile both in their length and breadth ; the power of 

 recovery, according to that author, being greater in proportion as the vessel is 

 nearer the heart. From this it follows that the pulmonary artery and aorta are 

 most liable to change in dimensions. As, however, any material alteration in 

 the size of the pulmonary artery and aorta might interfere with the proper 

 function of the semilunar valves situated at their orifices, it is curious to note 

 that the great vessels arise from strong and comparatively unyielding fibrous 

 rings. These rings (particularly the aortic one) are so dense as to be almost 

 cartilaginous in consistence, and Professor Donders* has lately discovered, that 

 they contain stellate corpuscles similar in many respects to those stellate and 

 spicate corpuscles, found in many forms of cartilaginous tumours. They have 

 been more or less minutely described by Valsalva,! Gerdy,:}: Dr John Reid,§ 

 and Mr W. S. Savory, || and merit attention because of their important relations 

 to the segments of the semilunar valves. The following description of the aortic 

 and pulmonic fibrous rings, has been drawn up chiefly from the examination of a 

 large number of human hearts. Each ring, as will be seen by a reference to 



* " Onderzockingen betrekkeligh den bouw van het menchclijke hart," in " Nederlandsch 

 Lancet" for March and April 1852. 



f Opera Valsalva, torn. i. p 129. 



+ Journal Complimentaire, torn. x. 



§ Cyc. Anat. and Phy. article " Heart," pp. 588, 589. London, 1839. 



11 Paper read before the Royal Society in December 1851. 



VOL. xxin. PART in. 9 u 



