1444 SUEFACE AND SUEGICAL ANATOMY. 



To understand the effect of lesions of the spinal medulla, it is necessary to be familiar 

 with the sensory and motor distributions of the various spinal segments (see Figs. 609, 

 p. 693, and 607, p. 688). Transverse lesions of the spinal medulla above the fifth cervical 

 spine (that is, above the nbro-cartilage between the fourth and fifth cervical vertebrae) 

 are quickly fatal, owing to paralysis of respiration, as the phrenic nerve arises mainly 

 from the fourth segment. In transverse lesions of the cervical enlargement the cutaneous 

 insensibility does not extend higher than a transverse line at the level of the second 

 intercostal space. The diagnosis of the particular segment involved is arrived at by 

 testing the motor and sensory functions of each segment. The sensory areas cor- 

 responding to the lower four cervical and the first two thoracic segments occupy the 

 upper extremities, and are placed in numerical order from the lateral to the medial side 

 of the limb. The sensory area corresponding to the second, third, and fourth cervical 

 segments occupy the occipital region of the scalp, the back of the auricle, and the 

 masseteric region, the whole of the neck, and the shoulders and upper part of the 

 chest down to a horizontal line at the level of the anterior end of the third intercostal 

 space. In a total transverse lesion of the spinal medulla in the thoracic region, the 

 superior limit of the anaesthesia is horizontal, and reaches to the level of the termina- 

 tions of the anterior rami of the spinal nerves which arise from the spinal segment 

 opposite the vertebral injury. Hence the superior limit of the anaesthesia is at a much 

 inferior level than that of the injured vertebra. For example, a fracture-dislocation at the 

 level of the eighth thoracic vertebra involves the origin of the tenth thoracic nerve which 

 ends at the level of the umbilicus. The sensory zone corresponding to the fifth thoracic 

 segment is at the level of the nipples, that of the seventh thoracic segment is at the level 

 of the xiphoid process, that of the tenth at the level of the umbilicus, while that of the 

 twelfth reaches down, anteriorly, to the superior border of the symphysis. The sensory 

 areas corresponding to the lumbar and sacral segments are seen in Figs. 627, p. 725, and 

 629, p. 733. 



THE UPPEE EXTEEMITY. 



THE SHOULDER. 



The bony landmarks of the shoulder must be systematically examined in all 

 injuries about that region. The medial extremity of the clavicle is prominent ; its 

 articulation with the sternum forms essentially a weak joint, which is liable to be 

 dislocated, especially from blows upon the lateral part of the shoulder which drive 

 the medial end of the clavicle forwards against the weak anterior sterno-clavicular 

 ligament. The body of the clavicle, subcutaneous throughout, is weakest at the 

 junction of its two curves ; it is in that region that the bone is so frequently 

 fractured as the result of force transmitted through it to the trunk. The dis- 

 placement of the lateral fragment varies according to whether the break takes 

 place medial or lateral to the coraco-clavicular ligament ; in the former case the 

 weight of the upper extremity, acting through the coraco-clavicular ligament, 

 pulls the lateral fragment downwards ; when the fracture is lateral to the ligament, 

 the lateral end of the clavicle rotates forwards, but there is no downward displace- 

 ment. The lateral end of the clavicle is on a plane posterior to its medial end, so 

 that the shoulder is braced backwards away from the thorax ; hence in fractures 

 of the clavicle, both medial and lateral to the coraco-clavicular ligament, the point 

 of the shoulder rotates forwards and medially. The acromio- clavicular articulation 

 is somewhat difficult to feel ; the groove which corresponds to it runs in the sagittal 

 direction, and lies 1J in. medial to the lateral border of the acromion, and im- 

 mediately lateral to a slight prominence upon the lateral extremity of the clavicle. 

 "When the acromio-clavicular joint is dislocated the clavicle almost invariably over- 

 rides the acromion, and the summit of the shoulder presents a somewhat conical 

 or " sugar-loaf " appearance. 



The tip of the acromion looks directly forwards, and lies a finger's breadth 

 lateral to and a little in front of the lateral extremity of the clavicle. The lateral 

 border of the acromion can readily be followed to its junction with the spine of the 

 scapula, and the latter to its root, which is situated on a level with the third 

 thoracic spine. The medial border of the acromion and the posterior border of the 

 lateral end of the clavicle meet at an angle into which the point of the finger can 



