THE LOWER EXTREMITY. 489 



and dorsal regions. The patient should bend the body forward, as by so doing the 

 space between the vertebrae posteriorly is increased. 



The needle used should be from 6.25 cm. (2^ in.) to 10 cm. (4 in.) long, ac- 

 cording to the age and size of the patient. It should be directed slightly toward the 

 median line and upward. In its entrance it pierces the muscles, then the ligamentum 

 subflavum, which passes from one lamina to the other, and finally the dura mater and 

 arachnoid. Failure is liable to occur either from the patient straightening the spine 

 when the puncture is made or from failure to enter the spinal membranes owing to 

 pushing the dura in front of the cannula. A needle enters more readily and surely 

 than does a small trocar with its cannula. The shoulder formed by the cannula, partic- 

 ularly if not well made, is apt to push the tough dura ahead of it instead of puncturing. 



Laminectomy. The laminae pass from the transverse and articular processes 

 to the spinous processes. On each side of the median line the erector spinae muscles 

 form thick masses and the spinous processes lie in the groove between them. Hence, 

 in doing a laminectomy, the depth at which the laminae lie is apt to be found much 

 greater than is expected. An incision is first made directly on the spinous processes 

 and continued down on each side to the laminae. With a chisel-like periosteal ele- 

 vator the attachments of the muscles and periosteum are detached from the base of 

 the spinous processes and laminae as far out as the transverse processes. The bleed- 

 ing from the muscles is controlled by packing. The laminae may be divided with a 

 saw inclined inward or the supraspinous, interspinous, and subflava ligaments may be 

 divided, the spinous processes cut close to their base and removed, and finally the 

 laminae removed with bone forceps. When the laminae are removed the dura mater 

 is found separated from the bone by fat and connective tissue. The veins here en- 

 countered may bleed freely but cease on pressure being made. If necessary the dura 

 may be opened, in which case the portion of the body toward the head may be low- 

 ered to prevent too great loss of cerebrospinal fluid. The roots of the spinal nerves 

 will be found passing out laterally and should if possible be avoided. If the poste- 

 rior or sensory root is divided it has the same tendency to re-unite as do sensory 

 nerves elsewhere, but division of the anterior root causes permanent motor paralysis. 

 The dura and other structures are then sutured without drainage. 



THE LOWER EXTREMITY. 



GENERAL CONSIDERATIONS. 



The lower extremity is designed to bear the weight of the body and serve as a 

 means of locomotion. 



It is composed of a pelvic girdle, thigh, leg, and foot. 



The pelvic girdle serves as the medium of connection of the lower extremity to 

 the trunk in the same manner as does the shoulder-girdle for the upper extremity. 



We saw that prehension, characterized by mobility, was the distinguishing feat- 

 ure of the upper extremity and that the shoulder-girdle was composed of two bones, 

 was loosely joined to the trunk, and held the upper extremity out away from it. 



The lower extremity on the contrary has two functions, it must bear the weight 

 of the body and must move this weight around from place to place; hence strength is 

 essential and a less amount of mobility suffices. To meet these changed conditions the 

 lower extremity differs in its construction from the upper in the following respects: 



1. The pelvic girdle is composed of one bone the innominate instead of two. 



2. It also forms a part of a complete bony ring instead of being incomplete 

 posteriorly. 



3. It is more firmly joined to the trunk. 



4. The hip-joint is placed closer to and in more intimate relation with the trunk 

 than is the shoulder. 



5. The bones of the lower extremity are Heavier and stronger than those of the 

 upper. 



6. The joints are larger and stronger, but their movements are not so extensive. 



7. The muscles are coarser and their functions are not so highly specialized. 



