THE MODIFICATIONS OF THE HEART. 89 



C'^.us the heart remains in its primitive state of a simple, con- 

 tractile, undivided tube. 



In the Ganoidei, the Elasmobranchii, and the Amphibia, 

 the walls of the enlarged commencement of the cardiac aorta, 

 called the bulbus aortce, contain striped muscular fibre, and 

 are rhythmically contractile. 



The Ganoidei and Elasmobranchii possess, not merely the 

 ordinary semilunar valves, at the junction between the ventri- 

 cle and the cardiac aorta, but a variable number of additional 

 valves, set, in transverse rows, upon the inner wall of the 

 aortic bulb. 



The change of position which the heart and the great ves- 

 sels of the highest Vertebrata undergo during embryonic life 

 is exceedingly remarkable, and is repeated as we ascend in the 

 series of adult vertebrates. 



At first, the heart of a mammal lies under the middle of 

 the head, immediately behind the first visceral arches, in which 

 the first pair of aortic arches ascends. As the other pairs of 

 aortic arches are developed the heart moves backward ; but 

 the fourth pair of aortic arches, by the modification of one of 

 which the persistent aorta is formed, lies, at first, no farther 

 back than the occipital region of the skull, to which, as we 

 have seen above, the fourth pair of visceral arches belongs. 

 As the two pairs of cornua of the hyoid belong to the second 

 and the third visceral arches, the larynx is probably developed 

 within the region of the fourth and fifth visceral arches ; hence, 

 the branches of the pneumogastric, with which it is supplied, 

 must, originally, pass directly to their destination. But, as 

 development proceeds, the aortic arches and the heart become 

 altogether detached from the visceral arches and move back, 

 until, at length, they are lodged deep in the thorax. Hence 

 the elongation of the carotid arteries ; hence also, as the 

 larynx remains relatively stationary, the singular course, in 

 the adult, of that branch of the pneiunogastric, the recurrent 

 laryngeal, which primitively passed to the laryngeal region 

 behind the fourth aortic arch, and consequently becomes 

 drawn out into a long loop — the middle of it being, as it 

 were, pulled back, by the retrogression of the aortic arch into 

 the thorax. 



77ie Sloodr Corpuscles. — Corpuscles are contained in the 

 blood of all Vertebrata. In Amphioxus they are all of one 

 kind, colorless and nucleated. The genus Leptocephalus, 

 among the Teleostd, is said to possess the same peculiarity ; 



