194 MYOLOGY. 



INTERCOSTALES. 



{Externi and interni.) 



(PL. III. 16, 17.) 



These fill up the spaces between the ribs, are external and 

 internal, and usually seventeen in number, consisting of flat 

 bands, included in, and intersected by, many broad aponeurotic 

 slips. The external ones are thickest above, the internal below, 

 the former terminating at the costal cartilages, between which 

 the latter extend. 



Origin. — From the posterior border of each rib. 



Insertion. — The anterior border of each succeeding rib, the 

 fibres of the external ones extending downwards and backwards, 

 those of the internal ones downwards and forwards. The external 

 terminate at the costal cartilages, while the internal do not 

 extend above the angles. 



Relation. — Externally with the serratus magnus, superficialis 

 and transversalis costarum, and latissimus dorsi ; internally with 

 the pleura. 



Action. — They are inspiratory muscles, drawing the ribs for- 

 ward, the first rib being rendered a fixed point by the contraction 

 of the scalenus. They thus aid the diaphragm in its action. 



Pectoral Region. 



In this region we describe two groups of muscles, the 'pectoral 

 and the sternal, the latter being deep-seated. 



Pectoral Group. 



This group consists of four pairs of muscles, which are largely 

 developed in the horse, and all of which arise from the external 

 surface of the sternum and its appendages, and become attached 

 to the pectoral limb, upon which they, as a group, assist in sus- 

 pending the body, also in moving the limb, and in respiration. 

 They are differently described by different authorities ; Percival 

 and Leyh describe three pairs, Chauveau two, but as the latter 

 divides each of them into two parts, his consideration ultimately 

 agrees with the one adopted here. We describe four pairs of 

 pectoral muscles : — 



Pectoralis transversus. I Pectoralis anticus. 



PectoralLs magnus. | Pectoralis parvus. 



