MUSCLES OF THE TRUNK. 235 



tracted at its two extremities, situated in front of the preceding, arising 

 from the sternum, directed forwards and outwards towards the scapulo- 

 humeral angle, and afterwards reflected upwards and backwards on the 

 anterior border of the shoulder, which it follows to near the cervical angle 

 of the scapula. 



Structure and Attachments. It is formed of very large fleshy fasciculi, 

 analogous to those of the sterno-trochineus, which originate, by their 

 inferior extremities, from the sides of the sternal keel and the cartilages of 

 the first three or four ribs. They follow the direction of the muscle, and 

 terminate, one above the other, on a short aponeurosis which covers the 

 supraspinatus, and is confounded with the external aponeurosis of the 

 scapula (Fig. 114, 14). 



Relations. In its axillary portion, this muscle responds, inwardly, to 

 the costo-sternalis, the first sternal cartilages, and the corresponding inter- 

 costal muscles : outwards, to the sterno-trochineus and sterno-aponeuroticus. 

 In its prescapular portion, it is in relation, outwardly, with the mastoido- 

 humeralis and trapezius; inwardly, with the subscapulo-hyoideus, the 

 scalenus, and the angularis of the scapula ; behind, with the supraspinatus 

 which is separated from it by the external scapular aponeurosis. 



Action. This muscle is a congener of the sterno-trochineus, and pulls 

 the scapula backwards and downwards. It is also a tensor of the scapular 

 aponeurosis. 



DIFFERENTIAL CHARACTERS OF THE MUSCLES OF THE AXILLARY REGION IN OTHER 

 THAN SOLIPED ANIMALS. 



"With regard to the superficial pectoral, it is remarked that in the Ox, Sheep, and 

 Pig, the sterno-lmmeialis is small end less distinct from, the sterno-aponeuroticus than in 

 Solipeds ; and that in the Dog and Cat, the sterno-aponeuroticus is very thin and 

 narrow. 



In the deep pectoral there is found, in the Ox, a sterno-prescapularis scarcely distinct 

 from the sterno-trochineus, and which does not extend beyond the interior extremity of 

 tl>e supraspinatus. In the Sheep, this muscle is quite confounded with the sterno- 

 trochineus. In the Pig, the sterno-presoapularis somewhat resembles that of the Horse. 

 Its inferior extremity only covers the first chondro-sternal articulation; the superior 

 extremity is more voluminous. With regard to the sterno-trochineus, it terminates on 

 the summit of the great trochanter, after detaching a short branch to the tendon of the 

 coraco-humeralis. The sterno-prescapulails of the Dog is very feeble, and terminates 

 with the principal muscle on the hurnerus. 



COSTAL REGION. 



In each costal region we find fifty-four muscles, which concur, more or 

 less directly, in the respiratory movements. These muscles are : 1, The 

 great serratus ; 2, The costo-transversalis ; 3, Seventeen external intercostals ; 

 4, Seventeen internal intercostals; 5, Seventeen supercostals ; 6, The 

 triangularis sterni. 



Preparation. 1. Place the subject in the second position. 2. Remove the fore- 

 limb and all the muscles attaching it to the trunk, by sawing through the scapula as 

 shown in figure 10, in order to expose the great serratus and costo-transversalis muscles ; 

 finish the dissection of the former by taking away all the yellow fibrous tissue which 

 covers its posterior dentations. 3. Study the external intercostals and the supercostals, 

 after removing the great oblique muscle of the abdomen, the serrati muscles, the 

 common intercostal (ilio-costalis), and the ilio-spinalis. 4. Excise some external inter- 

 costals in order to show the corresponding internal ones. 5. The triangularis is dis- 

 sected on another portion, which is obtained in separating the sternum from the thorax 

 by sawing through the sternal ribs a little above their inferior extremity. 



