524 A MANUAL OP ANATOMY 



so gives rise to the joint-cavities, the synovial membranes and ligaments being 

 formed by the surrounding mesenchyme. 



The muscular tissue, originally derived from the muscle-plates of the 

 somites, or perhaps from the mesenchyme which forms the core of the limb- 

 bud, becomes differentiated into the limb-muscles. 



The prolongations from the spinal nerve-segments become differentiated 

 into the limb-nerves. 



Position of the Limbs. — At first the fore- and hind-limbs are disposed with 

 their longitudinal axes parallel with the longitudinal axis of the trunk. As 

 the limbs undergo elongation and division into their ultimate component 

 parts, a change in their position takes place. They become bent in a ventral 

 direction, so that their longitudinal axes are now at right angles to the longi- 

 tudinal axis of the trunk. The radial or preaxial side of the antibrachium 

 and the pollex, on the one hand, and the tibial or preaxial side of the leg, and 

 the hallux, on the other, are directed towards the cephalic region ; the palmar 

 and plantar surfaces of the hands and feet are directed towards the trunk; 

 the elbows are directed outwards and slightly backwards; and the knees are 

 directed outwards and slightly forwards. 



Towards the end of the sixth week a second change in the position of the 

 limbs takes place, and their permanent condition is thereby brought about, 

 in which their longitudinal axes coincide with the longitudinal axis of the 

 trunk. This change consists in a rotation of each limb round its longittidinal 

 axis. In the case of the fore-limbs the rotation takes place in an outward 

 direction, but in the case of the hind-limbs the rotation is inwards. 



The permanent results of these rotations are as follows: Fore-Limbs [Ex- 

 ternal Rotation): (i) The radial or preaxial side of the antibrachium and 

 the pollex are directed outwards; (2) the elbow becomes entirely posterior; 

 and (3) the ventral or flexor aspect of the limb is now anterior, the extensor 

 aspect being posterior. Hind-Limbs [Internal Rotation): (i) The tibial or 

 preaxial side of the leg and the hallux are directed inwards; (2) the knee 

 becomes entirely anterior; and (3) the ventral or flexor aspect of the hmb is 

 now posterior, the extensor aspect being anterior. 



From the preceding changes it will be evident that, in the adult, the anterior 

 aspect of the upper limb corresponds to the posterior aspect of the lower Umb; 

 the outer aspect of the upper hmb to the inner aspect of the lower limb; and 

 so on. 



GUIDE TO THE LOWER LIMB. 



Gluteal Region. — The landmarks having been studied, the skin is to be re- 

 moved by making the following incisions : one extending from the level of the 

 spine of the fifth lumbar vertebra outwards along the iliac crest as far as the 

 position of the body will allow, and another extending from the fifth lumbar 

 spine downwards in the middle line as low as the coccyx, after which it is 

 prolonged outwards and downwards to the outer side of the thigh at a point 

 about 5 inches below the great trochanter. The skin having been reflected, 

 the gluteal cutaneous nerves are to be exposed in the following situations : 

 (i) line of origin of gluteus maximus ; (2) iliac crest ; (3) outer and lower part 

 of gluteus maximus ; and (4) lower border of gluteus maximus. The super- 

 ficial fascia should now be removed, and the deep fascia examined. It will 

 be seen that, in passing from the gluteus medius to the gluteus maximus, it 

 firmly straps down the upper border of the latter muscle to the former. 

 When the gluteus maximus has been reflected, the fascia will be seen to divide 

 into two laminae, which embrace and give insertion to rather more than the 

 upper half of that muscle. 



