THE DISSECTION OF THE BACK 73 



length of the intertransverse intervals. Additional rounded fasciculi may 

 be observed passing between the accessory processes ; they are termed 

 the interaccessorii. In the thoracic region intertransverse muscles poorly 

 developed are found only in the lower three or four spaces. In the cervical 

 region they are present in pairs and will be examined subsequently. 



The interspinous muscles help to bend the vertebral column backwards. 

 The intertransverse muscles bend it towards their own side. Both groups 

 are supplied by the posterior rami of the spinal nerves. 



Levatores Costarum. The elevators of the ribs constitute 

 a series of twelve fan-shaped muscles, which are classified as 

 muscles of the thorax, but .they are exposed when the 

 longissimus and ilio-costalis are removed, and therefore 

 should be examined now.. They pass from the transverse 

 processes to the ribs. The first muscle of the series springs 

 from the tip of the transverse process of the last cervical 

 vertebra, and, expanding as it proceeds downwards and 

 laterally, it is inserted into the outer border of the first rib, 

 immediately beyond the tubercle. Each of the succeeding 

 muscles takes origin from the tip of a thoracic transverse 

 process, and is inserted into the outer surface of the rib 

 immediately below, along a line extending from the tubercle 

 to the angle. The levatores costarum are muscles of in- 

 spiration. They are supplied by the anterior rami of the 

 thoracic nerves. 



Posterior Rami of the Sacral Nerves. The posterior 

 rami of the sacral nerves are very small. The upper four 

 will be found emerging from the posterior sacral foramina j 

 the fifth appears at the lower end of the sacral canal. 



To expose the upper three^ the multifidus, covering the 

 upper three sacral apertures, must be carefully removed. 

 Each of the three nerves will be found dividing in the 

 usual manner into a medial and lateral division. 



The medial divisions are very fine, and end in the 

 multifidus. 



The lateral divisions are somewhat larger, and join 

 together to form a looped plexus upon the dorsum of the 

 sacrum. The plexus communicates, above, with the lateral 

 division of the posterior ramus of the last lumbar nerve and, 

 below, with the posterior ramus of the fourth sacral nerve. 

 Branches proceed from the loops to the surface of the sacro- 

 tuberous ligament (O.T. great sacro-sciatic). Finally, they 

 become superficial by piercing the glutaeus maximus muscle, 

 and they supply a limited area of skin over the gluteal 



