THORAX AND ABDOMEN OF THE HORSE 7 



which it is distinguished with difficulty ; indeed, the levator may be 

 regarded as a strong and specialised part of the intercostal. 



It should be noted that the degree of development of the external 

 intercostal muscle varies greatly in different parts of the chest as well 

 as in different individuals. In some intercostal spaces the muscular 

 fibres may fail to reach from one rib to the next, a thin aponeurosis 

 taking their place. In the ventral part of the last few intercostal 

 spaces also the muscle may be very defective. 



Dissection. — Remove the external intercostal muscles from several 

 interspaces in order to show the underlying internal muscles. Take 

 care not to injure the intercostal vessels and nerves that, near the 

 vertebral column, will be found lying between the two muscles and 

 about midway between two ribs. 



Mm. intercostales inteuni. — The seventeen internal intercostal 

 muscles differ from the external intercostals in that their fibres run 

 with a ventro-cranial slope, and each muscle is continued between the 

 costal cartilages. In the neighbourhood of the sternum and costal arch, 

 therefore, the internal muscles can be seen before the external inter- 

 costals have been removed. 



Near the vertebral column the internal intercostal muscles become 

 thin and may disappear. 



Aa. intercostales. — The origin of the seventeen intercostal arteries 

 cannot be determined at the present time. When the interior of the 

 thorax is being examined at a later stage of the dissection, it will be 

 found that the first intercostal arises from the deep cervical artery ; the 

 second, third and fourth (possibly also the fifth) from the supreme 

 intercostal artery ; while all the rest spring from the thoracic portion of 

 the aorta. 



Each intercostal artery divides into a small dorsal and a larger 

 ventral branch. The dorsal branch (ramus dorsalis) divides into (1) a 

 ramus spinalis that enters the vertebral canal by an intervertebral 

 foramen, supplies small twigs to the membranes enclosing the spinal 

 cord, and ends by joining the ventral spinal artery ; and (2) a ramus 

 muscularis that pierces the dorsal end of an intercostal space to supply 

 the muscles and skin dorsal to the plane on a level with the bodies of 

 the thoracic vertebrae. 



The ventral branch (ramus ventralis) is now under examination. 

 At first it lies between the external and internal intercostal muscles 

 about midway between two ribs. Later it runs between the internal 

 intercostal muscle and the pleura in a groove on the caudal border of 

 a rib. 



Each intercostal artery ends at the ventral extremity of an inter- 



