x.] THE CIRCULATING SYSTEM. 429 



anastomosing branches with those which take the blood from 

 the gills to the aorta. This we- have seen to be the case in 

 the Tadpole at a certain stage of its development. Indeed, 

 the primitive condition in Fishes is that of a complete con- 

 tinuity of each arch from the heart to the dorsal aorta, and 

 it is only as embryonic development proceeds, that each arch 

 becomes broken up into a network of capillaries. Examples 

 are not wanting of the persistence of this primitive condition 

 throughout life in some of the arches, as e.g. in Monopterus 

 and Lepidosiren (Fig. 368). 



Thus we see that two different and divergent modes of 

 respiratory circulation may be developed from the same 

 starting-point : the primitive series of aortic arches in the 

 one case sending out posteriorly extending branches for a 

 lung-circulation; in the other case, themselves bieaking up 

 to form a gill-circulation. 



A third mode of respiratoiy circulation, that by the skin, 

 may supplement the others as in Batrachians, where a 

 large artery is given off from the heart to the cutis. As we 

 shall see in Lesson XII., both pulmonary and gill respiration 

 may co-exist, as in certain Tailed- Batrachians. 



In man the heart is placed at a considerable distance from 

 the cornua of the os hyoides, but in Fishes in close proximity 

 to their enlarged representatives the branchial arches. 



At first, however, even in him, these parts are adjoined, 

 and it is the subsequent displacement of the heart which 

 causes certain diverging and obscuring structural conditions. 



Thus the great aortic arch of Mammals is the remnant of 

 the vascular arches going to the fourth embryonic visceral 

 skeletal 1 arch, i.e. to the third behind the mandibular skeletal 

 arch. Yet the thyroid artery is placed much in front of 

 (i.e. preaxial to) the great aorta, though from the distribution 

 of the thyroid artery it probably corresponds with an artery 

 going to the fifth visceral skeletal arch, i.e. the third branchial 

 arch of Fishes, and perhaps to the fourth and fifth also. 



A similar explanation may be given of the fact noticed in 

 the last lesson the bending back of a nerve to the larynx 

 called the recurrent laryngeal. This nerve originally passed 

 behind the fourth aortic arch (one side of which persists as 

 the great arch of the aorta), without any marked curvature. In 

 the adult this nerve descends a long distance and then returns 

 upwards at a sharp angle, passing on the left side round the 

 arch of the aorta, and on the right side round the subclavian. 



For a description of these visceral skeletal arches see pages 95 and 143. 



