vi . AETEKIAL SYSTEM 403 



the body. Tims Lin- ri^ht pulmonary artery conies to supply the dorsal 

 side of both lungs, and the left artery the ventral side of both lungs. 

 Each lung in other words receives a supply of blood from loth 

 pulmonary arteries and this illustrates an initial step towards the 

 condition in A m in \\hciv the right and left sides of the air-bladder 

 the honioloo-uc of the right lung are supplied with blood directly by 

 a typical ri^ht ami left pulmonary artery. In the Actinopterygian 

 fishes apart from Amia the pulmonary artery has disappeared entirely 

 from development and the air-bladder receives its blood-supply by 

 secondary connexions with the dorsal aorta and its branches. 



CAROTID ARTERIES. As has already been indicated the great 

 longitudinal arteries ventral and dorsal aortae are prolonged for- 

 wards into the region of the head as the carotid arteries ventral and 

 dorsal. Of these the latter, receiving as it does, in the case of the more 

 primitive Vertebrates, blood which has been oxygenated by passing 

 through the gills, becomes the more important and is responsible 

 for supplying blood to the brain. 



The ventral carotids are found from the Lung-fishes onwards 

 perhaps in correlation with the reduction of the first two aortic arches. 

 They are known commonly under the name external carotid ( = lingual 

 artery of Amphibia) and supply blood to the ventral side of the head, 

 though cases are known amongst animals no longer having func- 

 tional gills (certain Mammals) in which they take over the blood- 

 supply of the brain also. 



It will be convenient to consider first the carotids as they occur 

 in the development of the Amniota. The simplest condition is 

 found in an ordinary Lizard (Lacerta) where they are seen as 

 apparent prolongations forwards of the aortic root and of the ventral 

 aorta respectively. This simple arrangement becomes in other Amniota 

 modified during the course of development in different ways, of which 

 the following are the chief. In various Lizards e.g. Chameleons 

 and Monitors (and the same holds for the great majority of Amniota 

 Fig. 187, A) the portion of aortic root between aortic arches III 

 and IV disappears during development, the consequence being that 

 arch III comes to form the posterior portion of the internal carotid 

 artery, becoming drawn out in the process of growth so as to be in 

 line with the front part of that artery derived from the aortic root. 

 The paired part of the ventral aorta from which the third arch was 

 given off now becomes the common carotid artery (c.c). The un- 

 paired portion of the ventral aorta, from which the common carotids 

 spring, is known as the primary carotid and in the long-necked 

 monitors this becomes much elongated, the growth in length of the 

 neck taking place in the region between aortic arches III and IV 

 (Fig. 187, A). 



In the European Grass-snake (Tropidonotus, Fig. 187, B) an 

 anastomosis is formed between the two internal carotids just behind 

 the head (Fig. 187, B, an) and, correlated with this, the right common 

 carotid as a rule disappears except for a small branch at its hinder 



