GENERAL PLAN OF THE ARTERIAL SYSTEM. 



721 



a regression whereby it becomes situated in the thorax, and in this migration it 

 carries backward (downward) with it the puhnonary arteries and the arch of the 

 aorta and produces an elongation of the carotids. As a result of the regression of 

 the aortic arch, the lateral branches which arose from the anterior portions of the 

 dorsal aorta and were distributed to the cervical segments of the body become sepa- 



Segtnental 

 Dorsal aorta arteries 



Gut-tube 



Fig. 677. 



Dorsal aortic trunks 



Gill-cleft 



Head arteries 

 Oropharynx 



Head arteries 



Heart 



Truncus 



arteriosus 



Ventral aortae 



Third aortic (gill) bow 



Diagram showing fundamental arrangement of arteries in fish, supposed to be viewed 

 from the side and above; shaded tube represents digestive canal with its gill-clefts in front 

 surrounded by series of six aortic bows and behind by segmental arteries. 



rated from their origins as far down as the branch to the seventh cervical segment, 

 which becomes the adult subclavian artery, but having developed anastomoses with 

 one another, so that a longitudinal stem, running parallel with the internal carotid 

 and attached below to the subclavian, is formed, they appear in the adult as lateral 

 branches of that stem which is ternied the vertebral artery. Primarily there are no 

 longitudinal arteries in the body, with the exception of the carotids and the dorsal 

 aorta ; but just as the vertebral artery is formed in the neck by the anastomosis of 

 upwardly and downwardly directed branches from lateral vessels, so, too, in other 

 regions, such as the thoracic and abdominal walls, other longitudinal stems are 

 secondarily developed. 



The dorsal aorta throughout its course gives off with almost segmental regular- 

 ity lateral branches to the body-walls which form the intercostal and lumbar arteries, 

 the fifth lumbar branches becoming greatly enlarged to supply the lower limb, and 

 being termed the iliac arteries. Below the origin of these the aorta is represented 

 only by a comparatively slender vessel, the middle sacral artery, which is continued 

 to the tip of the coccyx, giving off lateral branches with a more or less distinct seg- 

 mental arrangement. The visceral branches which arise from the aorta do not retain 

 their original segmental arrangement as perfectly as do the branches to the body- 

 walls, but fuse to a very considerable extent, especially in the abdomen, to form a 

 small number of vessels which ramify to the A^arious portions of the digestive tract 

 and to the genito-urinary abdominal organs. 



Fig. 678. 



Aortic arch 



Left subclavian 

 Aorta 



Dorsal aortae 



Digestive 

 canal 

 Right 

 subclavian 



Pulmonary artery 



Internal carotids 



Ascending aorta 



':« Oropharynx 

 External carotids 



Ventral aortae 



Diagram showing derivation of arteries in man by modifications in preceding plan ; 

 left fourth aortic bow becomes aortic arch. 



It will be seen, therefore, that the arterial system consists of two fundamental 

 portions, a brarichial and a dorsal aortic portion. A classification of the vessels of 

 the adult according to such a plan would, however, result in considerable confusion, 

 since, owing to the secondary modifications which have occurred, it would necessi- 

 tate the separation into different groups of arteries which are closely related, and, 



46 



