100 THE ANATOMY OF THE HORSE. 



is inserted into the side of the sternum. Frequently some of its fibres 

 terminate on the 3rd and 4th costal cartilages, or on the aponeurosis 

 over the internal intercostal between these cartilages. 



Action. — Acting from its attachment to the 1st rib as its fixed point, 

 the muscle will exert a feeble inspiratory action. 



The External Intercostal Muscles (Plate 21). Each muscle of this 

 set occupies an intercostal space, extending from near the spine as far 

 as the lower extremities of the ribs. The muscular fibres of each are 

 fixed by their extremities to the margins of the ribs that bound the 

 intercostal space. They pass obliquely downwards and backwards ; and 

 may be considered as having their point of origin from the anterior rib> 

 and their insertion into the posterior rib. 



Directions. — In one or two of the spaces the external intercostal 

 should be removed (see Plate 21) in order to expose the internal 

 muscle, which will readily be distinguished by the different directions 

 of its fibres. 



The Internal Intercostal Muscles (Plate 21). These equal in 

 number the external set, one being lodged in each intercostal space. 

 They differ from the external set in that they are prolonged beyond 

 the lower extremities of the ribs to fill the interspaces of the costal 

 cartilages, while in the extreme upper part of the intercostal spaces they 

 are absent or much reduced in thickness. They differ, moreover, in the 

 direction of their fibres, which is oblique downwards and forwards ; and 

 each may be viewed as having its origin from the posterior rib and 

 cartilage, and its insertion into the anterior rib and cartilage, of the 

 space that it occupies. The inner surface of each is lined by pleura,, 

 but at present no attempt need be made to expose this. 



Action of the intercostal muscles. — The external set and the inter- 

 cartilaginous portions of the internal set are muscles of inspiration. 

 The interosseous portions of the internal set are muscles of expira- 

 tion. 



Directions. — In a few of the intercostal spaces the vessels and nerves m 

 should be exposed. They will be found at the hinder edge of the rib, 

 and should be followed upwards and downwards. 



Intercostal Arteries. There are seventeen intercostal arteries on 

 each side, one for each space. The first is derived from the superior 

 cervical artery; the second, third, and fourth from the dorsal artery or its 

 subcostal branch ; and the remaining thirteen from the posterior aorta. 

 Their points of origin will be seen in the dissection of the cavity of 

 the thorax. Each vessel on gaining the upper extremity of the inter- 

 costal space gives off a large dorso-spinal branch, and then descends 

 behind the rib, with the vein and nerve. The dorso-spinal artery sends 

 a branch into the spinal canal by the intervertebral foramen, and is then 

 expended in the muscles occupying the costo-vertebral groove at the side 



