THE MUSCLES OF THE THORAX 281 



sacral spines, with the multifidus, and the transverse processes of the sacral and 

 coccygeal vertebree. 



Insertion. — The lateral surface of the coccygeal vertebrae, 'except the first four. 



Action. — Acting with its fellow, to assist the preceding muscle in elevating the 

 tail; acting singly, to incline it to the same side. 



Structure. — This muscle appears to be a direct continuation of the multifidus 

 dorsi. The belly is fusiform and receives reinforcing fasciculi from the transverse 

 processes of the sacrum. This is succeeded by bundles of tendons, as many as four 

 lying alongside of each other. 



Relations. — Superficially, the lateral sacro-iliac ligament and the coccygeal 

 fascia; dorsally, the sacro-coccygeus dorsalis; ventrally, the intertransversales; 

 deeply, the vertebrae and a branch of the lateral coccygeal artery and accompanying 

 vein and nerve. 



4. Intertransversales caudse (Mm. intertransversarii caudae). — These con- 

 sist of muscular bundles which lie on the lateral aspect of the tail, between the 

 preceding muscle and the sacro-coccygeus ventrahs. They begin on the lateral 

 edge of the sacrum and occupy the spaces between the transverse processes, to which 

 they are attached. They are, however, not arranged in a strict segmental manner. 



Action. — Acting together, to fix the coccygeal vertebrae; acting singly, to assist 

 in lateral flexion. 



5. Sacro-coccygeus ventralis (Mm. sacro-coccygei ventrales).^ — This muscle 

 lies on the ventral aspect of the sacrum and coccyx. It is composed of two parts, 

 described by Bourgelat and the German anatomists as separate muscles. 



(a) The lateral part (M. coccygeus ventralis lateralis) is much the larger of 

 the two. It arises from the lateral part of the ventral surface of the sacrum, about 

 as far forward as the third foramen, and is inserted into the transverse processes 

 and ventral surface of the coccygeal vertebrae. 



(fe) The medial part (M. sacro-coccygeus ventralis medialis) arises from the 

 ventral surface of the sacrum medial to the preceding muscle and the first eight 

 coccygeal vertebrae, and is inserted into the ventral surfaces of the coccygeal ver- 

 tebrae. 



Action. — Acting together, to depress (flex) the tail; acting singly, to incline 

 it laterally also. 



Structure. — The lateral part has a somewhat compressed belly, and receives 

 bundles from the transverse processes of the coccygeal vertebrae. The medial part 

 is much smaller and shorter, reaching only about to the middle of the tail. 



Relations. — -Ventrally, the pelvic and coccygeal fasciae; dorsally, the sacrum, 

 coccygeal vertebrae, and the intertransversales; laterally, the sacro-sciatic liga- 

 ment, the coccygeus, and the coccygeal fascia; medially, its fellow, the recto- 

 coccygeus, and the middle coccygeal vessels. Branches of the lateral coccygeal 

 vessels and nerves lie between the lateral division of the muscle and the intertrans- 

 versales. 



Blood-supply. — Middle and lateral coccygeal arteries. 



Nerve-supply. — Coccygeal nerves. 



The Muscles of the Thorax 

 (musculi thoracis) 



These consists of seven muscles or sets of muscles, which are attached to the 

 thoracic vertebrae, to the ribs and their cartilages, and to the sternum. They are 

 muscles of respiration. 



1. Levatores costarum. — These constitute a series of small muscles which oc- 

 cupy and overlie the dorsal ends of the intercostal spaces. 



' Also known as the depressor coooygis. 



