254 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



epigastric veins. In their course the two internal mammary veins receive 

 branches from the diaphragm, chest wall, pericardium, etc. The precaval vein 

 next receives small branches from the thymus glands and adjacent muscles, and 

 at a level between the first and second ribs is seen to be formed by the union of 

 two large veins. These are the brachiocephalic or innominate veins. They are 

 the two precaval veins of embryonic stages which later unite to form the single 

 precaval vein of adult anatomy by the crossing over of the left vein to join the 

 right one (Fig. 59, p. 268). The branches of the two brachiocephalic veins are 

 identical, and only one need be followed, preferably the right one, since the right 

 side has not been touched in the previous dissection. The places of entrance 

 of the various tributaries' are, however, somewhat variable. 



Immediately anterior to the junction of the two brachiocephalics, opposite 

 the first rib, each of them receives on the dorsal side a large tributary. This is 

 located by dissecting on the dorsal side of the vein and lifting the vein. The main 

 part of the tributary can be traced into the cervical vertebrae; it is the vertebral 

 vgin and courses in the vertebrarterial canal, collecting from the brain and spinal 

 cord. Before it enters the brachiocephalic the vertebral is joined by the costocer- 

 vical vein, which comes from the muscles of the back, and receives branches also 

 from the chest wall on the inner surface of the first two ribs. The costocervical 

 vein may be picked up by turning the animal dorsal side up and, on the side 

 where the muscles were dissected, dissecting in the serratus ventralis and the 

 epaxial muscles. The communication of the vertebral and costocervical veins 

 with the brachiocephalic and with each other is variable and may not be as 

 described here. 



The brachiocephalic at the same place as the entrance of the veins just 

 described is seen to be formed by the union of two large veins, a lateral subclavian 

 and an anterior external jugular. The subclavian will be followed first. It 

 passes laterally in front of the first rib into the axilla, where it is known as the 

 axillary vein. Expose the axilla by cutting through the pectoral muscles near 

 the midventral line and at their insertion on the humerus. The pectoral muscles 

 sliould then be separated from the underlying serratus ventralis but should not 

 be removed. The stout white cords crossing the axilla are the nerves of the 

 brachial plexus and are not to be injured. Lymph glands will also be noted in 

 the axilla. The most medial tributary of the axillary vein is the large sub- 

 scapular vein which passes through the proximal part of the upper arm to the 

 dorsal side of the humerus and collects from various muscles of the upper arm 

 and shoulder, receiving also the posterior circumflex vein from the external sur- 

 face of the upper arm. The beginnings of the subscapular vein will be found in 

 the trapezius muscles. The axillary vein lateral to the entrance of the subscapu- 

 lar receives the small ventral thoracic vein from the medial portions of the pector- 

 al muscles. Lateral to this it receives the long thoracic vein, which runs caudad 

 along the inner surface of the pectoral muscles; and the thoracodorsal vein , which 



