DEVELOPMENT OF ARTERIES IN FORELIMB OF PIG. 



147 



of the forelimb could be rendered visible. This 

 has enabled the humerus, radius, and ulna to be 

 represented. The wrist and hand bones could 

 also be identified, but have not been included 

 in the picture. The scapula also has been 

 omitted. A knowledge of the position of the 

 skeletal parts renders identification of the vessels 

 more certain and easy. The vertebral, the 

 dorsal (this artery is the equivalent of the supe- 

 rior intercostal and profunda cervicis of human 

 anatomy), the thoraco-acromial, and internal 

 mammary need no comment except to say that 

 they are complete stems, all trace of the anas- 

 tomosing network whence they have come having 

 disappeared. The brachial has a characteristic 

 concavity directed caudally before it passes in 

 front of the lower extremity of the humerus. 

 This bend is present in all observed specimens 

 between 16 and 20 mm. The descending branch 

 of the brachial has been identified as the sub- 

 scapular artery. From this artery two branches 

 pass on the dorsal side of the humerus; the upper 

 one is the posterior circumflex, the lower is the 

 profunda accompanying the radial nerve. These 

 arteries are still very plexiform and the profunda 

 plexus links up with the plexiform radial artery. 

 Around the elbow are many plexiform branches 

 of the brachial, making a rich and abundant 

 cubital anastomosis. Many tiny plexiform 

 branches arise from the brachial in this part of 

 its course. At the lower extremity of the hu- 

 merus the brachial undergoes subdivision. Its 

 largest branch is the median artery, which ends 

 in four digital capillary meshes which embrace 

 the skeleton of the hand in a fine tracery of 

 capillaries. Between the digits communications 

 with dorsal vessels are apparent. Just before 

 these digital capillary meshes are formed, the 

 median artery itself expands into a wide plexi- 

 form mass. Just beyond the elbow the radial 

 artery arises. It retains much of the primitive 

 condition of all vessels. It passes toward the 

 radial side of the limb and becomes more diffuse. 

 Communications with the profunda artery are 

 apparent. Towards its termination two strands 

 can be identified. One strand becomes a capil- 

 lary mesh for the radial digit, the other communi- 

 cates with the digital mesh of the median, thus 

 suggesting a superficial palmar arch. Eventu- 

 ally, its terminal capillaries, like the other digital 

 capillaries, join with the marginal vein. 



The ulnar is a feeble plexiform artery and in 

 the adult pig does not get very far beyond its 

 present condition. A fourth division of the 

 brachial termination is the volar interosseous. 

 This exists before the median artery can be identi- 

 fied and after the median has appeared the volar 



for a period exceeds it in dimensions. At the 

 present stage the volar is the smaller and is 

 tucked between the bones of the forearm. It 

 ends in a ventral and dorsal division. The 

 ventral division anastomoses with the digital 

 branches of the median; the dorsal division 

 passes between the radius and ulna, proximal 

 to the carpals, and forms a dorsal digital capil- 

 lary meshwork. The dorsal interosseous can be 

 identified as a plexiform group, taking origin 

 from the volar interosseous and wandering 

 distally. Some of its terminal capillaries joining 

 the marginal vein are apparent. 



The marginal vein still forms a peripheral 

 boundary to the limb extremity. The ulnar 

 half is larger and more definite than the radial 

 half. Between the digits the continuity of this 

 venous channel is beginning to disappear and 

 examination of the extremities of the two radial 

 digits shows that the vein is beginning to sur- 

 round these digital rudiments, a smaller loop 

 passing ventral and a larger loop passing on 

 the dorsal side of the blunt digital end. Each 

 extremity of the marginal loop is continued along 

 the margin of the limb. The radial margin bears 

 the vena ccphalica. Into this there open venous 

 channels from the digits, for out of each dorsal 

 digital network a venous trunk arises and, pass- 

 ing obliquely over the dorsum of the extremity, 

 reaches the vena cephalica. The latter receives 

 numerous communications as it passes, to end 

 in the external jugular. 



On the ventral surface of the limb, in close 

 association with the median artery, is the median 

 vein. This arises out of digital capillaries and, 

 receiving numerous tributaries, runs along the 

 ventral aspect. It ends in the region of the 

 elbow chiefly in the vena cephalica, but also in 

 the vena basilica. The vena basilica, arising 

 as a continuation of the marginal vein, grows 

 larger as the trunk is reached and, bending 

 cephalad, lies ventral and slightly above the 

 subclavian artery. At this point it terminates 

 in the cardinal vein. The vena basilica is thus 

 directly continued as the subclavian vein, re- 

 ceiving numerous tributaries from the trunk 

 and body-wall, as well as tributaries from the 

 arm which represent the venae comites of the 

 artery. 



Figure 5 includes the aorta and pulmonary 

 arteries, with the cut ends left by the removal 

 of the heart. This specimen illustrates well 

 the three stages in the life history of the arteries 

 the capillary stage, the retiform stage, and the 

 final, definite tubular form. In comparison with 

 the veins, it is easily seen that these retain much 

 more of the primitive stages. 



