DISSECTION OF THE DOG 5 



been already noted in connection with the dissection of the pectoral muscles. 

 Very commonly the deltoid ramus arises from the omo-cervical trunk. 



A. THORACALis LATERALIS.— The lateral thoracic artery arises some 

 little distance from the border of the fii-st rib, supplies the pectoral muscles, 

 runs along the deep face of the deep pectoral muscle in company with the nerve 

 to the latissimus dorsi, and ends in the cutaneous muscle over the abdomen. 



Truncus omo-cervicalis.— The omo-cervical trunk is a branch of the 

 subclavian artery. Arising within the first rib, it passes medial to the axillary 

 artery and the brachial plexus to divide before long into the ascending cervical 

 and transverse scapular arteries. 



The ascending cervical artery (a. cervicalis ascendens) runs for a distance on 

 the medial surface of the brachio-cephalic muscle, and finally enters the 

 substance of this muscle. 



The transverse scapular artery (a. transversa scapulae) passes towards the 

 cranial border of the subscapular muscle and divides into several branches, 

 the main one of which accompanies the suprascapular nerve. 



N. PHRENiCTJS.— The phrenic nerve will be found between the axUlary vein 

 and artery dorsal to the omo-cervical arterial trunk. It arises by three roots 

 from the ventral divisions of the fifth, sixth, and seventh cervical nerves, and 

 leaves the present dissection by entering the thorax. Later, it mil be 

 traced to the diaphragm to which it carries motor impulses. 



Dissection. — Cut across the axillary vessels, close to the first rib. This 

 allows of a more complete inspection of the brachial plexus. 



Plexus brachialis. — The brachial plexus is a somewhat complex arrange- 

 ment of nerves formed by branches from the last three cervical and the first 

 two thoracic nerves. It is placed between the axillary vessels and the scalenus 

 muscles. 



Its branches are : Nerve to the brachio-cephalic muscle, n. swprascapularis , 

 n. suhscapularis, n. musculo-cutaneus , n. axillaris, n. radialis, n. medianus, 

 n. ulnaris, n. thoraco-dorsalis, nn. thoracales ventrales, and n. thoracalis longus. 



Most of these mil be examined in the dissection of the limb ; but the 

 following points should be noted at the present moment. The ventral thoracic 

 nerves (nn. thoracales ventrales) are mainly distributed to the pectoral muscles. 

 One of them supplies the skin and cutaneous muscle on the side of the chest 

 and abdomen. The thoraco-dorsal nerve (n. thoraco-dorsalis) suppUes the 

 latissimus dorsi muscle. 



If the subject is a male the external genital organs should next be dissected. 

 Partes genitales externje. — The male external genital parts consist 

 of the scrotum, the penis, and the prepuce. 



The scrotum is a membranous bag with a double cavity in which the testes 



B 3 



