DEVELOPMENT OF ARTERIES IN FORELIMB OF PIG. 143 



cation of the future pattern, as has been stressed by Streeter (1918); that out of 

 this vascular net there will be determined particular paths, in accordance with the 

 postulates of Thoma (1893); and lastly, that such a dynamic view of the vascular 

 development is not in conflict with any phylogenetic view of the order of blood-ves- 

 sels, since the dynamic, equally with the static, is a heritage of the past. 



For the opportunity of making this investigation I am indebted to Dr. L. H. 

 Weed and Dr. G. L. Streeter, who have so generously placed their experience and 

 the hospitality of their departments at my disposal. To Dr. C. H. Heuser, who 

 taught me the technique involved, and to Dr. F. R. Sabin, who allowed me to study 

 her collection of excellent material, my best thanks are due. 



METHOD AND MATERIAL. 



The vascular arrangement was made obvious by the introduction of india ink 

 and by clearing according to the method of Spalteholz. This method has ad- 

 vantages over that of serial sections and modeling, which reveals only blood-vessels 

 that happen to be stuffed with red corpuscles. The collapsed vessels are apt to 

 be ignored, while those that happen to be full are given exaggerated value and ele- 

 vated to the rank of a special designation. Apart from these objections, which 

 will vary in their justness according to the state of the material investigated, it 

 can be urged that the injection method is necessary to study the capillary stage of 

 blood-vessels, which the other method has almost invariably failed to establish. If 

 the ink is introduced gently into the umbilical artery while the heart is yet beating, 

 its presence will excite the heart to vigorous contractions and produce an altogether 

 beautiful picture of the vascularity. Since the ink is distributed by the cardiac 

 contractions, it naturally follows the distribution of the blood and thus gives in 

 these tiny embryos a faithful portrait of the relative dimensions of the blood-vessels. 

 Such has been the method adopted and it has been successful in embryos as small 

 as 4 or 5 mm. 



The embryos were obtained from an adjacent abattoir immediately after 

 evisceration of the carcasses. They were removed from the uterus and placed in 

 warm salt solution and immediately injected. The smaller ones were fixed in Bouin's 

 fluid, the larger in formalin, dehydrated, and cleared in oil of wintergreen. A very 

 great number of embryos of each of the stages about to be described were studied. 



ANATOMY OF THE BLOOD-VESSELS OF THE FORELIMB. 



The aorta, which is strongly curved, gives off the brachiocephalic trunk, which 

 in turn subdivides into a right subclavian, two common carotids, and a left sub- 

 clavian, which arises from the aortic arch just above the common carotid. Each 

 subclavian gives off a dorsal, a vertebral, and a deep cervical branch, which arise 

 close together or from a common trunk. From the same common trunk, or from 

 the dorsal artery, arise the intercostal artery to the second intercostal space and 

 the subcostal artery, which supplies the third, fourth, and fifth intercostal spaces. 

 The dorsal artery emerges through the dorsal extremity of the second interspace 

 and divides into dorsal and cervical branches. The dorsal branch runs deep to the 

 muscles of the back which it supplies; the cervical branch passes anteriorly to the 



