148 DEVELOPMENT OF ARTERIES IN FORELIMB OF PIG. 



SURVEY OF SPECIAL LITERATURE. 



Before summarizing these observations it may be of advantage to review the 

 results of other investigators upon which our knowledge of the development of the 

 blood-vessels of the arm has been based. 



Dohrn (1889) showed that the subclavian artery belongs to the system of 

 segmental branches of the aorta. He described these as vertebral arteries having 

 two branches, a ventral branch, winch supplies the lateral and ventral muscula- 

 ture, and a dorsal branch, which supplies the central nervous system and spinal 

 musculature. The subclavian is one of the vertebrals. Mollier (1894) showed 

 that more than one of these segmental arteries was concerned in the blood supply 

 of the pectoral fin of the Selachian. 



Miiller (1904) described the arterial supply of the forelimb in an Acanthias 

 embryo 20 mm. long. The arteries of the extremities are four in number and are 

 given off by the aorta to the lateral body-wall. Each of these arteries sends a 

 branch to the extremity and dissolves into capillaries in the proximal part of the 

 limb. Out of these, one particular branch survives and becomes the main artery of 

 the limb; the remaining branches from the aorta to this root-net dwindle away. 

 Mollier (1895) showed a comparable arrangement for Lacerta muralis. Miiller (1904) , 

 in a 4-mm. Lacerta embryo, traced three segmental arteries into the limb-mass. 



Svensson (1908) investigated the subclavian in Lacerta muralis. He showed 

 that the limb supply develops from three segmental arteries, the sixth, seventh, 

 and eighth. These anastomose and enter the limb-mass. This plexus was de- 

 scribed by Miiller as the "plexus arteriosus axillaris," and this name was adopted 

 by Svensson. Out of this plexus, by the dwindling of one part and an increase 

 in another, the chief artery of the limb arises. The primordium of the brachial 

 begins as a plexus, which Svensson calls the "plexus arteriosus brachialis." Hoch- 

 stetter (1890a) studied the subclavian in the bird. He showed that the main 

 vessel of the limb, arising as a twig from the aorta, later joins, through a secondary 

 branch, with the third aortic arch and that the primitive subclavian forms out of 

 these. The same condition holds for the crocodile and chelonia. C. G. Sabin 

 (1905) and Rabl (1906) have investigated the chick, Evans (1909) the duck 

 embryo. To the four periods in the history of the bird's subclavian, Evans adds a 

 still earlier one. 



Evans: 



(1) Period of capillary outgrowth from the aorta forming a primary limb plexus not influ- 

 enced in its arrangement by metamerism. 



Rabl, Miiller: 



(2) Period of multiple segmental subclavians, a condition resulting from the atrophy of 

 all the capillaries in the preexisting plexus not at segmental points. 



(3) Period of establishment of the primary subclavian artery from the persistence of one 

 of the pairs of segmental subclavians, i. e., the eighteenth. 



Hochstetter, Sabin: 



(4) Period of double arterial supply through a contemporary existence of dorsal and newly 



arisen ventral subclavians. 



(5) Period of enlargement of the permanent channels, the secondary subclavian, and 



coincident atrophy and disappearance of the primary vessels. 



