238 THE MUSCLES. 



Form Situation. This muscle, flattened above and below, elongated 

 from before to behind, and dentated at its external or superior border, is 

 situated in the thoracic cavity, above the sternum and the cartilages of the 

 true ribs. 



Attachments. It is fixed, by its internal border, on the superior face 

 of the sternum, to the ligamentous cord which circumscribes it outwardly. 

 It has its movable insertion on the cartilages of the sternal ribs, the first 

 excepted, by means of digitations from its external border. 



Structure. It is formed of strongly aponeurotic muscular fasciculi, which 

 are directed from the internal to the external border. 



Relations. Inwardly, with the pleura ; outwardly, with the cartilages to 

 which it is attached, the internal intercostals, and the internal thoracic vein 

 and artery. 



Action. The triangularis of the sternum concurs in expiration by 

 depressing the costal cartilages. (Leyh asserts that if the fixed point 

 be the sternum, this muscle pulls the ribs forwards, and so widens the 

 thoracic cage ; but if the fixed point is the ribs, the sternum will be raised 

 and the thoracic space diminished.) 



DIFFERENTIAL CHARACTERS IN THE MUSCLES OF THE COSTAL REGION IN OTHER 

 THAN SOLIPED ANIMALS. 



The muscles of the costal region cannot "be the same in number in all the domes- 

 ticated animals ; the intercostals and supercostals, for instance, must vary in number 

 with that of the ribs. Beyond this, the differences are slight. In the Ox, the great 

 serratus is very extensive, and the portion which passes to the posterior triangular 

 surface of the scapula is readily distinguished from the anterior by its diminished 

 thickness, the larger proportion of aponeurotic fibres it contains, and the flattened 

 tendon by means of which it is inserted. In the Pig, it is remarked that the internal 

 intercostals are prolonged, maintaining a certain thickness, to near the vertebral column. 



COMPARISON OF THE THORACIC MUSCLES OF MAN WITH THOSE OF THE DOMESTICATED 



ANIMALS. 



The muscles of the axillary region, the costal region, and the diaphragm are desig- 

 nated in Man as the thoracic muscles. 



The pectoral muscles are distinguished into great and small. The great pectoral 

 corresponds to the superficial pectoral of the Horse. It is attached, on one side, to the 

 inner two-thirds of the clavicle, the anterior face of the sternum, and the cartilages of 

 the first six ribs; on the other, to the anterior border of the bicipital groove and, by a 

 fibrous expansion, to the aponeurosis of the arm. The costal fasciculi are distinctly 

 separated from the clavicular and sternal fasciculi. 



The small pectoral, which corresponds to the deep pectoral of animals, is inserted, on 

 the one part, into the external face of the third, fourth, and fifth ribs , on the other part 

 by a tendon to the anterior border of the coracoid process. 



In Man, there is found a muscle which does not exist in animals ; this is the sub- 

 clavius, a very slender fasciculus situated beneath the clavicle, and attached to the 

 cartilage of the first rib and the external portion of the lower face of the clavicle (see 

 Fig. 117, 5). 



The great serratus does not show any distinct aponeurosis on its surface ; it arises 

 from the eight first ribs, and its digitations are grouped into three principal fasciculi. 



Lastly, in Man the internal intercostals are prolonged to the vertebral column by 

 small muscles, named the subcostals (or intracostals). 



INFERIOR ABDOMINAL REGION. 



The lateral and inferior walls of the abdominal cavity are formed by a 

 wide musculo-aponeurotic envelope, which rests, by its periphery, on the 

 sternum, ribs, lumbar vertebrse, ilium, lumbo-iliac aponeurosis, and the 

 pubis. This envelope is concave on its superior surface, and results from 

 the assemblage of four pairs of large membranous muscles arranged in 



