Miscellaneous, 235 



and which establishes a more and more free communication between 

 the hypo- and the epibranchials towards the point where these latter 

 issue from the gill. 



In a general way the constitution of the definitive type of the 

 circulatory system is realized by the direct inosculation of the 

 bulbar arches with the origin of the arteries arising from the 

 epibranchials, by the medium of the interbranchials, and the disap- 

 pearance of the entire branchial circulatory system with the gill 

 itself. 



We will now see what takes place in the case of each of the 

 branchial arches. 



First arch. — The short vascular plexus which represents the inter- 

 branchial establishes a direct communication between the first 

 bulbar arch and the carotid lingual, which then constitutes the 

 termination of the former. The interbranchial becomes the carotid 

 gland. The connective between the first and second arches dis- 

 appears. 



Second arch. — An open anastomosis is formed, by means of the 

 interbranchial, between the second bulbar arch and the origin of 

 the aorta. The connective between the second and third arches 

 atrophies. 



Third and fourth arches. — In the case of these arches a more 

 complex transformation takes place. 



By means of the third interbranchial and of the connective be- 

 tween the third and fourth arches, which persists, the third bulbar 

 arch becomes continuous with the afferent vessel of the lung, or 

 pulmonary artery, which chiefly arises from the epibranchial of the 

 fourth arch. 



The fourth bulbar arch, which is merely a subdivision of the third, 

 loses its function and atrophies. 



The pulmonary artery of the adult, constituted by the various 

 sections which I have just enumerated (third bulbar arch, third 

 interbranchial, and the connective between the third and fourth 

 arches), gives rise, on a level with the third gill, to a vessel which 

 may be ascribed to the epibranchial of that gill, and which forms 

 the extremely interesting branch of the pulmonary artery known as 

 the cutaneous respiratory. 



When the pulmonary sac begins to develop it receives a vessel 

 fed by the epibranchial of the fourth arch and also by the connec- 

 tive between the third and fourth. The result of this arrangement 

 is that this pulmonary vessel, which is none other than the pul- 

 monary artery, receives already aerated blood, mingled with a small 

 quantity of venous blood coming from the interbranchial anasto- 

 moses. Thus in reality at the outset this artery is a nutritive 

 vessel like the other arteries. 



Towards the close of larval life, when the lung is a functional 

 respiratory organ, the artery maintains its nutritive role, but, iu 

 addition to this, the blood that it contains is aerated afresh and the 

 pulmonary veins bring back pure arterial blood to the heart. 



