THE MUSCULO-CUTANEOUS NERVES. 797 



side of the leg and the integument of the dorsum of the foot. It passes forward 

 between the Peronei muscles and the Extensor longus digitorum, pierces the deep 

 fascia at the lower third of the leg on its front and outer side, and divides into 

 two branches. This nerve in its course between the muscles gives off muscular 

 branches to the Peroneus longus and brevis, and cutaneous filaments to the 

 integument of the lower part of the leg. 



The internal branch of the musculo-cutaneous nerve passes in front of the 

 ankle-joint, and divides into two branches, one of which supplies the inner side 

 of the great toe, the other, the adjacent sides of the second and third toes. It 

 also supplies the integument of the inner ankle and inner side of the foot, 

 communicating with the internal saphenous nerve, and joining with the anterior 

 tibial nerve, between the great and second toes. 



The external branch, the smaller, passes along the outer side of the dorsum of 

 the foot, and divides into- two branches, the inner being distributed to the con- 

 tiguous sides of the third and fourth toes, the outer to the opposed sides of the 

 fourth and fifth toes. It also supplies the integument of the outer ankle and 

 outer side of the foot, communicating with the short saphenous nerve. 



The branches of the musculo-cutaneous nerve supply all the toes excepting 

 the outer side of the little toe, and the adjoining sides of the great and second 

 toes, the former being supplied by the external saphenous, and the latter by the 

 internal branch of the anterior tibial. It frequently happens, however, that 

 some of the outer branches of the musculo-cutaneous are absent, their place 

 being then taken by branches of the external saphenous nerve. 



Surgical Anatomy. The lumbar plexus passes through the Psoas muscle, and, therefore 

 in psoas abscess any or all of its branches may be irritated, causing severe pain in the part to 

 which the irritated nerves are distributed. The genito-crural nerve is the one which is most 

 frequently implicated. This nerve is also of importance, as it is concerned in one of the princi- 

 pal reflexes employed in the investigation of diseases of the spine. If the skin over the inner 

 side of the thigh just below Poupart's ligament, the part supplied by the crural branch of the 

 genito-crural nerve, be gently tickled in a male child, the testicle will be noticed to be drawn 

 upward through the action of' the Cremaster muscle, supplied by the genital branch of the same 

 nerve. The same result may sometimes be noticed in adults, and can almost always be produced 

 by severe stimulation. This reflex, when present, shows that the portion of the cord from which 

 the first and second lumbar nerves are derived is in a normal condition. 



The anterior crural nerve is in danger of being injured in fractures of the true pelvis, since 

 the fracture most commonly takes place through the ascending ramus of the os pubis, at or 

 near the point where this nerve crosses the bone. It is also liable to be injured in fractures 

 and dislocations of the femur, and is likely to be pressed upon, and its functions impaired, 

 in some tumors growing in the pelvis. Moreover, on account of its superficial position, it 

 is exposed to injury in wounds and stabs in the groin. When this nerve is paralyzed, the 

 patient is unable to flex his hip completely, on account of the loss of motion in the Iliacus ; 

 or to extend the knee on the thigh, on account of paralysis of the Quadriceps extensor cruris ; 

 there are complete paralysis of the Sartorius and partial paralysis of the Pectineus. There is 

 loss of sensation down the front and inner side of the thigh, except in that part supplied by 

 the crural branch of the genito-crural, and by the ilio-inguinal. There is also loss of sensation 

 down the inner side of the leg and foot as far as the ball of the great toe. 



The obturator nerve is of special surgical interest. It is rarely paralyzed alone, but occa- 

 sionally in association with the anterior crural. The principal interest attached to it is in con- 

 nection with its supply to the knee ; pain in the knee .being symptomatic of many diseases in 

 which the trunk of this nerve, or one of its branches, is irritated. Thus it is well known that 

 in the earlier stages of hip-joint disease the patient does not complain of pain in that articu- 

 lation, but on the inner side of the knee, or in the knee-joint itself, both these articulations 

 being supplied by the obturator nerve, the final distribution of the nerve being to the knee- 

 joint. Again, the same thing occurs in sacro-iliac disease : pain is complained of in the knee- 

 joint or on its inner side. The obturator nerve is in close relationship with the sacro-iliac 

 articulation, passing over it, and, according to some anatomists, distributing filaments to it. 

 Again, in cancer of the sigmoid flexure, and even in cases where masses of hardened faeces are 

 impacted in this portion of the gut, pain is complained of in the knee. The left obturator 

 nerve lies beneath the sigmoid flexure, and is readily pressed upon and irritated when disease 

 exists in this part of the intestine. Finally, pain in the knee forms an important diagnostic 

 sign in obturator hernia. The hernial protrusion as it passes out through the opening in the 

 obturator membrane presses upon the nerve and causes pain in the parts supplied by its periph- 

 eral filaments. When the obturator nerve is paralyzed, the patient is unable to press his knees 

 together or to cross one leg over the other, on account of paralysis of the Adductor muscles. 



