lOQ Dr Allen Thomson on the Vascular System 



and opposite to each of these secondary buds a new branch 

 of vessel is formed by the passage of the blood, directly across 

 the primary stalk, from the outgoing to the returning vessel. 

 As the new buds of the gills become longer, these communi- 

 cating branches between the primary vessels are produced along 

 with them ; new communicating branches are thrown out in 

 their course, while, at the same time, new buds are formed on 

 the primary stalk of the gill. There are generally about thirty 

 of these leaflets on the gills of the larva, at the time that they 

 have attained their full development, which, according to 

 Rusconi, is about the 40th or 45th day ; the vessels are then 

 still more minutely ramified on the surface of the leaflets, and 

 as they are almost quite transparent, the circulation of the 

 blood through them forms a truly beautiful spectacle. In this 

 animal, therefore, as in the fish, by tracing the development of 

 these vessels, it is easy to perceive that the branchial arteries and 

 veins are only subdivisions in the course of the aorta itself. 



At the period when the gills are fully developed, the distri- 

 bution of the vessels rising from the heart is the following (Fig, 

 9) : The three foremost pairs only of the branchial vascular 

 arches convey blood to the gills {in). At the root of each gill the 

 arterial or outgoing vessel communicates directly, by a short 

 branch, with the vein or returning vessel, so that a considerable 

 portion of the blood propelled into the branchial arteries, along 

 with the whole of that in the fourth or posterior pair of arches, 

 which gives no branches to the gill, is carried directly into the de- 

 scending aorta (r s). From the communicating branch at the root 

 of the foremost gill there arises a small artery (^), which is distri- 

 buted on the parts surrounding the hyoid bone. The parts near 

 the temporal bone receive an artery from the root of the second 

 gill (/), and the vertebral artery is given off" near the place 

 where the whole of the branchial arches unite to form the roots 

 of the descending aorta (r). The anterior extremity, like the 

 pectoral fin of the fish, receives its vessels from the mammary 

 artery (?<), which arises along with the coeliac, mesenteric, &c. 

 from the descending aorta. • 



While the external branchiae are developed, the internal 

 arches upon which they are supported become firmer and more 

 cartilaginous. They are attached anteriorly to the hyoid bone. 



