Chap, xni.] CIRCULATION. 203 



development of every vertebrate. In these days it is merely to 

 utter a commonplace to recall that the heart of every vertebrate 

 appears first of all, in the embryon, under the form of a simple 

 contractUe vesicle, and represents tolerably well, in its ulterior 

 development, first, the cardiacal type of the fish, then that of 

 the reptile. 



Further on we shall see that the anatomical muscular elements 

 are distinguished into muscles of the life of relation and muscles 

 of the nutritive life. The first constitute the muscular masses 

 which obey the will ; the others foi-m the contractile elements of 

 the viscera not subject to that influence. Now, by exception, 

 the heart, the organ of the organic and independent life of the 

 will, is constituted by muscular fibres of the animal life, by fibres 

 called striated. This is the case among mammifers, birds, and 

 reptiles. Nevertheless these cardiacal muscular fibres are not 

 absolutely identical with those of the muscles of the animal life. 

 They are thinner, have a granulous aspect ; besides, they have a 

 reciprocity of ramification and anastomosis. Striation, but of a 

 very fijie kind, is also seen on the cardiacal muscles of many in- 

 vertebrates, especially insects, spiders, and crustaceans : in short, 

 among the arthropods. There likewise the muscular fibres have 

 a granulous texture and a more sombre hue. 



The nervous apparatus of the heart is also altogether peculiar. 

 By and by we shall find that the peripheric part of the nervous 

 system is composed of afierent or motory fibres, of efferent or 

 sensitive fibres, and lastly of a nervous network having some 

 special anatomical features, and called the system of the sympa- 

 thetic nerves. Now the heart, already singular by the nature of 

 its muscular fibres, is also innervated in a particular fashion. It 

 receives equally sympathetic fibres with ganglionary expansions 

 and smooth fibres of the animal life. Finally, the distribution 

 of all these nervous fibres is not made uniformly among the 

 muscular fibres. If* employing a suitable ^sodic solution, we 

 render transparent considerable portions of the auricles and the 

 ventricles, we find therein few or no nerves. It is in proximity 



