240 FASCIA AND MUSCLES OF THE HORSE 



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 inferior (Mm. sacro-coccygei ventrales; depressor 

 coccygis). — This muscle lies on the ventral aspect of the sacrum and coccyx. 

 It is composed of two portions, described by Bourgelat and the German anatomists 

 as separate muscles. 



(a) The outer portion (M. coccygeus ventralis lateralis) is much the larger 

 of the two. It arises from the outer part of the ventral surface of the sacrmn, about 

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

 and ventral surface of the coccygeal vertebrae. 



(b) The innsr portion (M. sacro-coccygeus ventralis meclialis) arises from 

 the ventral surface of the sacrum internal to the preceding muscle and the first 

 eight coccygeal vertebrae, and is inserted into the ventral surfaces of the coccj^geal 

 vertebrae. 



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

 it laterally also. 



Structure. — The outer portion has a somewhat compressed belly, and receives 

 bundles from the transverse processes of the coccygeal vertebrae. The inner por- 

 tion 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 iDctween the outer division of the muscle and the intertrans- 

 versales. 



Blood-supply. — Middle and lateral coccygeal arteries. 



Nerve-supply .—Coccygesd nerves. 



The Muscles of the Thorax 



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

 thoracic vertebrae, to the ribs and their cartilages, and to the sternum. Func- 

 tionally, they are muscles of respiration. 



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

 and overlie the upper ends of the intercostal spaces. 



Origin. — -The transverse processes of the thoracic vertebrae. 



Insertion. — The external surfaces of the upper ends of the ribs posterior to the 

 vertebral origin. 



Action. — To draw the ribs forward in inspiration. 



Structure. — Arising by tendinous fibers, each muscle passes backward and out- 

 ward and expands at its insertion. Some fibers pass over one ril:) and are inserted 

 on a succeeding one. At the first and last spaces the muscle cannot be distin- 

 guished from the external intercostal, of which it is in reality onlj" a specially de- 

 veloped part. 



Relations. — -Superficially, the longissimus dorsi; deeply, the ribs, internal 

 intercostal muscles, and the intercostal vessels and nerves. 



Blood-supply. — Intercostal arteries. 



Nerve-supply. — Intercostal nerves. 



2. External intercostals (Mm. intercostales externi). — Each of these oc- 

 cupies an intercostal space, from the levatores to the sternal extremity of the rib. 

 They do not occupy the intercartilaginous spaces. 



Origin. — The posterior borders of the ribs. 



