ARTERIAL SYSTEM. 529 



auricular Wiills, wliich at first pass uninterruptedly the one into the other, 

 grows iuto the bases of the valves, wliich thus become in the main muscular 

 projections of the walls of the heart. As the wall of the ventricle thickens, 

 the muscular trabecuhe, connected at one end with the valves, remain at the 

 other end united with the ventricular wall, and form special bands passing 

 ])etween the two. The valves on the other hand lose their muscular 

 nttachment to the aui'icular walls. This is the condition permanent in 

 Ornithorhynchus. In higher Mammalia the ends of the muscular bands 

 inserted into the valves become fibrous, from the development of inter- 

 muscular connective tissue, and the atrophy of the muscular elements. 

 The fibrous parts now form the chordie tendiaeae, and the muscular tlie 

 musculi papillares. 



The sinus venosus in Mammals becomes completely merged into the 

 right auricle, and the systemic division of the truncus arteriosus is appa- 

 rently not homologous with that in Birds. 



In the embryos of all the Craniata the heart is situated very far 

 forwards in the rej^ioti of the head. This position is retained in Pisoes. 

 In Amphibia the heart is moved further back, while in all the 

 Araniota it gradually shifts its position first of all into the region 

 of the neck and finally passes completely within the thoracic cavity. 

 The steps in the change of position may be gathered from figs. 109, 

 111, and 118. 



Bibliography of the Heart. 



(492) A. C. Bernays. "Entwicklungsges^hichte d. Atrioventriciilarklappen." 

 Morphol. Jahrhnch, Vol. 11. 1876. 



(493) E. Gasser. " Ueber d. Entstehung d. Herzeiis beim Hiihn." Archiv f. 

 ■mikr. Anat., Vol. xiv. 



(494) A. Thomson. "On the development of the vascular system of the f(Etus 

 of Vertebrated Animals," Edinb. New Phil. Journal, Vol. ix. 1830 and 1831. 



(495) M. Tonge. "Observations on the development of the semilunar valves 

 of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans, clix. 1869. 



Vide also Von Baer (291), Eathke (300), Hensen ([82), Kolliker (298), Gotte (296), 

 and Balfour (292). 



Ai'terial System. 



In the embryos of Vertebrata the arterial system consists of a 

 forward continuation of the truncus arteriosus, on the ventral side of 

 the throat (fig. 363, abr, and 364, a), which, with a few exceptions 

 to be noticed below, divides into as many branches on each side as 

 there are visceral arches. These branches, after traversing the visceral 

 arches, unite on the dorsal side of the throat into a common trunk on 

 each side. This trunk (figs. 363 and 364) after giving off one (or more) 

 vessels to the head (c' and c) turns backwards, and bends in towards 

 the middle line, close to its fellow, immediately below the notochord 

 (figs. 21 and 116) and runs backwards in this situation towards the 

 end of the tail. The two parallel trunks below the notochord fuse very 

 early into a single trunk, the dorsal aorta (figs. 363, ad, and 364, a ). 

 There is given off from each collecting trunk from the visceral 

 arches, or from the commencement of the dorsal aorta, a subclavian 



B. E. ir. 34 



