Surface Anatomy of the Back 


Bones.—The only subcutaneous parts of the vertebral column are the apices of the spinous processes. These are distinguishable at the bottom of a furrow which runs down the middle line of the back from the external occipital protuberance to the middle of the sacrum. In the cervical region the furrow is broad and ends below in a conspicuous projection caused by the spinous processes of the seventh cervical and first thoracic vertebræ. Above this, the spinous process of the sixth cervical vertebra sometimes forms a projection; the other cervical spinous processes are sunken, but that of the axis can be felt. In the thoracic region the furrow is shallow and during stooping disappears, and then the spinous processes become more or less visible; the markings produced by them are small and close together. In the lumbar region the furrow is deep and the situations of the spinous processes are frequently indicated by little pits or depressions, especially when the muscles in the loins are well-developed. In the sacral region the furrow is shallower, presenting a flattened area which ends below at the most prominent part of the dorsal surface of the sacrum, i. e., the spinous process of the third sacral vertebra. At the bottom of the sacral furrow the irregular dorsal surface of the bone may be felt, and below this, in the deep groove running to the anus, the coccyx. 
The only other portions of the vertebral column which can be felt from the surface are the transverse processes of the first, sixth, and seventh cervical vertebrae. 

Muscles.—The muscles proper of the back are so obscured by those of the upper extremity (Fig. 1211) that they have very little influence on surface form. The Splenii by their divergence serve to broaden out the upper part of the back of the neck and produce a fulness in this situation. In the loin the Sacrospinales, bound down by the lumbodorsal fascia, form rounded vertical eminences which determine the depth of the spinal furrow and taper below to a point on the dorsal surface the sacrum. The continuations of the Sacrospinales in the lower thoracic region form flattened planes which are gradually lost on passing upward. 




Bony Landmarks.—In order to identify any particular spinous process it is customary to count from the prominence caused by the seventh cervical and first thoracic; of these the latter is the more prominent. The root of the spine of the scapula is on a level with the tip of the spinous process of the third thoracic vertebra, and the inferior angle with that of the seventh. The highest point of the iliac crest is on a level with the spinous process of the fourth lumbar, and the posterior superior iliac spine with that of the second sacral.  





FIG. 1212– Diagram showing the relation of the medulla spinalis to the dorsal surface of the trunk. The bones are outlined in red. 

Level of body of No. of nerve. Level of tip of spine of Level of body of No. of nerve. Level of tip of spine of 

Level of body of No. of nerve. Level of tip of spine of Level of body of No. of nerve. Level of tip of spine of

C.1 C.1
T.8 T.9 7 T.
2 2
9 10 8

3 1 C. 10 11 9
3 4 2
12 10
4 5 3 11 L. 1 11
5 6 4
2
6 7 5 12 3

8 6
4 12
7 T. 1 7
5
T. 1 2 1 T.
S. 1
2 3
L. 1 2
3 4 2
3
4 5 3
4 1 L.
5 6 4
5
6 7 5
C. 1
7 8 6 L. 2



The transverse process of the atlas is about 1 cm. below and in front of the apex of the mastoid process. The transverse process of the sixth cervical vertebra is opposite the cricoid cartilage; below it is the transverse process of the seventh and occasionally a cervical rib.  





FIG. 1213– Sagittal section of vertebral canal to show the lower end of the medulla spinalis and the flum terminale.. First and fifth lumbar vertebra. Second sacral vertebra. 1. Dura mater. 2. Lower part of subarachnoid cavity. 3. Lower extremity of medulla spinalis. 4. Filum terminale internum, and 5. Filum terminale externum. 6. Attachment of filum terminale to first segment of cooccyx.  






FIG. 1214– Scheme showing the relations of the regions of attachment of the spinal nerves to the vertebral spinous processes. 

Medulla Spinalis.—The position of the lower end of the medulla spinalis varies slightly with the movements of the vertebral column, but, in the adult, in the upright posture it is usually at the level of the spinous process of the second lumbar vertebra (Fig. 1212); at birth it lies at the level of the fourth lumbar. 
The subdural and subarachnoid cavities end below opposite the spinous process of the third sacral vertebra (Fig. 1213).