538 



DISSECTION OF THE BACK. 



Dissectton 

 of costal 

 muscles. 



Levatores 

 costarum : 



attach- 

 ments. 



The first. 



Longer 



elevator 



muscles. 



Use. 



Outer 



intercostal 



muscle. 



Dissection. 



Dorsal 

 nerve has 



posterior 



and anterior 

 branches. 



Intercostal 

 artery. 



Dissection. The posterior part of the wall of the thorax may be 

 examined before the body is again turned. By removing, opposite 

 the ribs, the ilio-costalis and longissirnus dorsi, the small levatores 

 costarum will be uncovered. The hinder part of the external 

 intercostal muscles will be denuded at the same time. 



The LEVATORES COSTARUM are twelve small fan-shaped muscles, 

 which are connected with the hinder parts of the ribs. Each, 

 except the first, arises from the tip of the transverse process of a 

 dorsal vertebra, and is inserted, the fibres spreading out, into the 

 upper border of the rib beneath, from the tuberosity to the angle. 

 The muscles increase in size from above down, and their fibres have 

 the same direction as the external intercostal layer. 



The first is fixed above to the transverse process of the last cervical 

 vertebra, and below to the outer border of the first rib. Some of 

 the fibres of the lower muscles are continued beyond one rib to that 

 next succeeding : these longer slips have been named levatores 

 longiores costarum. 



Action. These muscles have but little influence in elevating the 

 ribs ; and their principal use appears to be in extending and bending 

 laterally the spine. 



The EXTERNAL INTERCOSTAL MUSCLE is continued backwards 

 along the ribs as far as the tuberosity, where it joins the elevator 

 muscle. Beneath the muscle are the intercostal nerve and artery. 



Dissection. To trace the anterior and posterior primary 

 branches of the dorsal nerves to their common trunk, the elevator 

 of the rib and the external intercostal muscle are to be cut through 

 in one or more spaces. The intercostal artery with its posterior 

 branch is laid bare by this proceeding. 



The DORSAL NERVES split in the intervertebral foramina into 

 anterior and posterior primary branches. 



The posterior branches are directed backwards, internal to the 

 superior costo-transverse ligament ; aud their distribution has 

 been seen in the foregoing dissection. 



The anterior named intercostal, are continued between the ribs to 

 the front of the chest : their anatomy has been learnt in the 

 dissection of the thorax and upper limb. 



The INTERCOSTAL ARTERY has an almost exact correspondence 

 with the dorsal nerve in its branching and distribution. 



SECTION IV. 



THE SPINAL COED AND ITS MEMBRANES. 



Cord is con-: THE spinal cord (medulla spinalis) gives origin to the spinal 

 spinal canal, nerves, and is lodged in the canal bounded by the bodies and 

 invested by neural arches of the vertebrae. It is invested by prolongations 

 membranes. o f ne membranes of the brain, which form sheaths around and 

 support it. 



