ARTERIAL SYSTEM. 643 



and deserve to be noticed. In the primitive state the ventricular walls 

 have throughout a spongy character ; and the auriculo-ventricular valves are 

 simple membranous projections like the auriculo-ventricular valves of Fishes. 

 Soon however the spongy muscular tissue of both the ventricular and 

 auricular walls, which at first pass uninterruptedly the one into the other, 

 grows into the bases of the valves, which thus become in the main muscular 

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

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

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

 between the two. The valves on the other hand lose their muscular 

 attachment to the auricular 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 chordae tendinea?, and the muscular the 

 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 region of the head. This position is retained 

 in Pisces. In Amphibia the heart is moved further back, while 

 in all the Amniota 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, in, and 118. 



BIBLIOGRAPHY of the Heart. 



(492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen." 

 Morphol. Jahrbuch,^o\. II. 1876. 



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

 mikr. Anat., Vol. xiv. 



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

 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), Rathke (300), Hensen (182), Kolliker (298), Gotte (296), 

 and Balfour (292). 



Arterial System. 



In the embryos of Vertebrata the arterial system consists of 

 a forward continuation of the truncus arteriosus, on the ventral 



41 2 



