NERVES OF MAMMALIA. 169 



short way along the scalenus anticus ; as it sinks deeper, it gives 

 a filament to the pectoralis major, passes over the aortic arch and 

 trunk of the vagus in entering the thorax, passes along the 

 anterior mediastinum, and then along the pericardium to the left 

 side of the diaphragm. The right phrenic crosses the subclavian, 

 or trunk of the brachial artery, in entering the thorax, and 

 supplies the right half of the diaphragm. A small branch of 

 the anterior division of the fifth cervical, a large branch of that 

 of the sixth, a still larger one of the seventh, and a smaller 

 contribution from the first and second dorsal nerves combine to 

 form the axillary plexus, prior to which are sent off nerves to 

 the scalenus anticus, subscapularis, teres major, and latissimus 

 dorsi. From the plexus is continued a branch beneath the 

 triceps, which quickly radiates small filaments, one of the largest 

 of which is continued along between the radius and ulna ; a 

 second branch passes along the inner side of the triceps to the 

 olecranon ; a third branch goes between the hind border of the 

 scapula and the triceps outward and forward, it supplies the 

 infraspinatus and deltoid, and ends in the periosteum and skin at 

 the fore part of the humerus. Many small twigs are sent to the 

 subscapularis muscle. The hindmost and strongest branch goes 

 obliquely outward and backward, giving filaments to the latis- 

 simus dorsi, and bends over the chest to the sternum, along the 

 side of which it distributes itself to the serratus magnus and con- 

 tiguous muscles attached to the ribs ; it answers to the ( external 

 o 



thoracic nerve.' There are thirteen pairs of dorsal nerves, each 

 dividing into a dorsal and intercostal part. The dorsal division 

 bends over the rib-neck in the anterior vertebras, and over the 

 lengthening diapophysis in the posterior ones, and subdivides into 

 a superficial and deep part ; the latter supplies the spinales, 

 interspinales, and the fascia of the muscles of the back ; the 

 superficial nerves contribute to the longissimus dorsi, and 

 levatores costarum, in their way to the skin of the back and its 

 muscles. The ventral divisions of these nerves are less distinctly 

 subdivided into external and internal fasciculi than in quadru- 

 peds. The first intercostal sends a communicating branch to the 

 axillary plexus, before its normal distribution, as in the other 

 intercostals, to the muscles so called, which are perforated toward 

 the sternum by the branches going to the ventral integument. 

 The nerves answering to lumbar and sacral of Quadrupeds divide 

 into dorsal and ventral fasciculi. The former go to the inter- 

 transversales, spinales, interspinales, sacrolumbalis, and longis- 

 simus dorsi; and to the superincumbent fascia and tegument. 

 There are intercommunicating filaments between the dorsal divi- 



