254 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



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

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

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

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

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

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

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

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

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

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

 of the various tributaries are, however, somewhat variable. 



Immediately anterior to the junction of the two brachiocephalics, opposi 

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

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

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

 vein and courses in the vertebrarterial canal, collecting from the brain and spin 

 cord. Before it enters the brachiocephalic the vertebral is joined by the costoa 

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

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

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

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

 epaxial muscles. The communication of the vertebral and costocervical vei 

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

 described here. 



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

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

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

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

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

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

 should then be separated from the underlying serratus ventralis but should n 

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

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

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

 scapular vein which passes through the distal part of the upper arm to the dors 

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

 shoulder, receiving also the posterior circumflex vein from the external surfa 

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

 trapezius muscles. The axillary vein lateral to the entrance of the subscapul 

 receives the small ventral thoracic vein from the medial portions of the pectoi 

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

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



