210 MYOLOGY. 



and the lungs ; posteriorly, covered by peritoneum, it contacts 

 the liver, stomach, spleen, and the large colon. 



Action. — Essentially a muscle of inspiration. When it con- 

 tracts it becomes flattened, forces the abdominal viscera back- 

 wards, and so enlarges the thoracic cavity. This action is 

 antagonistic to one of the actions of the abdominal muscles. It 

 also assists in the expulsive efforts of defecation and parturition. 



Coccygeal Region. 



The muscles of this region, that of the tail, are enclosed in 

 strong fascia, which may be said to be continuous with the 

 approximate pelvic ligaments. We find one group containing 

 four muscles : — 



Erector coccygis. } Curvator coccygis. 



Depressor coccygis. \ Compressor coccygis. 



ERECTOR COCCYGIS. 



(Sacro-coccygeus Superior.) 



(PL. IIL 3L) 



Situated on the supero-lateral part of the tail, it consists of 

 bundles of fibresj intersected by tendons, which gradually diminish 

 in size from before backwards, each terminating in a short tendon. 



Origin. — The tuberous summits and lateral aspect of the 

 posterior sacral spines, and superior surface of the coccyx. 



Insertion. — To the coccygeal bones. 



Relation. — Internally with its fellow, and the coccygeal verte- 

 brae ; externally with the skin and fascia. 



Action. — When acting together to elevate the tail ; singly, to 

 draw it upwards and outwards. 



DEPRESSOR COCCYGIS. 



(Sacro-coccygeus Inferior.) 



(Pl. IIL 33.) 



Situated on the infero-lateral aspect of the tail, and similar to 

 the erector, but thicker. 



Origin. — Within the pelvis, from the inferior surface of the 

 sacrum, the inner surface of the sacro-sciatic ligament, and the 

 coccygeal vertebrse. 



