DISSECTION OF THE DOG 181 



Dissection. — The dissector should now turn to those muscles which lie 

 ventral to the thoracic and cervical vertebrae. 



M. longus colli. — This composite muscle, the long muscle of the neck, 

 stretches from the sixth or seventh thoracic vertebra to the atlas. It is 

 decidedly complicated in the arrangement of the numerous bundles which 

 compose it, and may be divided, for convenience of description, into thoracic 

 and cervical portions. The distinction of the two parts is readily made ; 

 for, though all the bundles of fibres pass in a cranial direction, those of the 

 thoracic portion slope lateralwards, while those of the cervical portion incline 

 towards the median plane. 



The bundles of the thoracic part arise from the ventral aspect of the bodies 

 of the first six or seven thoracic vertebra?, and are inserted into the transverse 

 processes of the last two cervical vertebrae. The fibres of the cervical part spring 

 from the transverse processes of the cervical vertebrae from the sixth or seventh 

 to the third, and are inserted into the ventral ridge of the vertebrae and the 

 ventral tubercle of the atlas. 



M. longus capitis. — The long muscle of the head arises from the transverse 

 processes of the cervical vertebrae from the sixth to the second, and also 

 occasionally from the lateral part of the aboral border of the wing of the atlas. 

 From this diffuse origin the fibres are gathered together and inserted into the 

 ventral surface of the basilar part of the occipital bone. 



M. rectus capitis ventralis. — The small ventral straight muscle of the 

 head lies dorsal to the foregoing, which should be reflected by making a trans- 

 verse incision on a level with the joint between the atlas and the occipital bone. 

 The origin of the straight muscle is from the ventral surface of the atlas, and its 

 insertion is into a depression on the occipital bone immediately aboral to the 

 insertion of the preceding muscle. 



M. rectus capitis lateralis. — Of about the same size as, and parallel to, 

 the ventral muscle, the lateral straight muscle of the head springs from the 

 ventral surface of the wing of the atlas, and is inserted into the jugular process 

 of the occipital bone. The occipital artery curves round its insertion. 



Dissection. — An examination of the pterygoid muscles and associated 

 structures should now be made on that side of the head on which the 

 mandible has been left intact. 



M. pterygoideus internus. — The internal pterygoid muscle forms a 

 considerable mass, the fibres of which pass from the pterygoid, palatine, and 

 sphenoid bones to the medial surface of the mandible ventral to the mandibular 

 foramen. At its insertion it is partly confused with the masseter muscle. 



Dissection. — The internal pterygoid muscle should be removed in order 

 that the external muscle of the same name may be examined. By this 



x 3 



