DISSECTION OF THE DOG 29 



Mm. inteecostales externi. — On each side of the chest there are twelve 

 external intercostal muscles. Each muscle passes from the border of one rib 

 to the adjacent border of the next, with fibres running in a caudo-ventral 

 direction. It will be observed that the first seven muscles terminate about 

 the level of union of the bony and cartilaginous segments of the ribs. 

 Frequently the eighth muscle proceeds farther and may almost reach the 

 sternum. The remaining muscles are also continued between the rib-cartilages, 

 but generally there is an interruption in their continuity at the end of the 

 bony segment of the rib. 



In a dorsal direction the external intercostal muscles reach the vertebral 

 column, but this cannot be determined as yet. 



Mm. interoostales interni. — The twelve internal intercostal muscles 

 differ from the external intercostals in that their fibres pass in a ventro-cranial 

 direction, and each muscle is continued along the entire length of an inter- 

 costal space. In the neighbourhood of the sternum, therefore, the internal 

 muscles can be seen before the removal of the external intercostals. 



Dissection. — Now carefully remove an internal intercostal muscle in order 

 to demonstrate the intercostal vessels and nerve. These will be found 

 lying along the caudal border of the rib, between the internal intercostal 

 muscle and the lining membrane of the chest. 



Aa. interoostales. — There are twelve intercostal arteries on each side of 

 the chest. At the present stage of dissection, the origin of the arteries cannot 

 be determined, but will be revealed when the wall of the thorax is examined 

 from within. Nor can the division of each artery into a dorsal and ventral 

 branch be exposed. At the present time it will be sufficient to note that the 

 vessel now displayed is the ventral branch (ramus ventralis) of an intercostal 

 artery. It travels down the caudal border of the rib between the corresponding 

 vein, which is cranial in position, and the intercostal nerve. 



The arteries of the first seven or eight intercostal spaces terminate by 

 anastomosing with branches from the internal mammary artery. The rest 

 anastomose with the musculo-phrenic artery, or are continued into the wall 

 of the abdomen. 



Lateral cutaneous branches (rami cutanei laterales) of the intercostal arteries 

 become superficial by piercing the overlying muscles about the middle of the 

 length of each interspace. 



Vv. interoostales. — Intercostal veins accompany the intercostal arteries 

 and are to be sought between the artery and the rib. 



Nn. interoostales. — Intercostal nerves course along the aboral border 

 of the arteries of the same name. Each constitutes the ventral branch 

 (ramus ventralis) of a thoracic spinal nerve. A lateral cutaneous branch 

 (ramus cutaneus lateralis) leaves each nerve at the same point as does the 

 like-named branch of the artery. 



