1408 CLINICAL AND TOPOGRAPHICAL ANATOMY 



incisions should always be made from behind forward. If more room is required, as in large 

 growths or in exploration of the ureter, the incision must be prolonged beyond the iliac crest, 

 the lumbo-ilio-inguinal incision of Morris. 



Viscera. — Several of these, which can be mapped in behind — viz., the Iddneys, 

 spleen, etc. — have been already mentioned (pp. 1375, 1379). 



The commencement of the trachea and oesophagus has been given in front as 

 corresponding to the sixth cervical vertebra. If examined from behind, this 

 point, owdng to the obliquity of the spines, would be a little lower down. The 

 trachea, about 12.5 cm. (5 in.) long, descending in the middle line, bifurcates 

 opposite to the interval between the third and fourth thoracic spines (or fourth 

 and fifth bodies). The bronchi enter the lungs at about the level of the fifth 

 thoracic spine, the right being the shorter, wider, and more horizontal. The 

 root of the lung is opposite to the fourth, fifth, and sixth dorsal spines, midway 

 between these and the vertebral border of the scapula. The structures in it are 

 the bronchus, pulmonary artery, two pulmonary veins, bronchial vessels, lymph- 

 atics, and nerves. The phrenic nerve is in front, the posterior pulmonary plexus 

 behind. On the right side the superior vena cava is in front, the vena azygos 

 (major) arching over the root at the level of the fourth thoracic vertebra. On 

 the left side the aorta arches over the root, and the thoracic aorta descends 

 behind it. The oesophagus, about 25 cm. (10 in.) in length, starting in the 

 middle line, curves twice to the left, at first gradually at the root of the neck; 

 from this point it tends to regain the middle line up to the fifth thoracic vertebra; 

 thence finalty turns again, and more markedly to the left, and passes through the 

 diaphragm opposite to the tenth, entering the stomach here or at the eleventh 

 thoracic vertebra (ninth or tenth thoracic spine). In the thorax this tube tra- 

 verses first the superior, then the posterior, mediastinum. At three spots, i. e., 

 its commencement, where it is crossed by the left bronchus, and at the cardiac 

 orifice, it presents narrowings. The relations of this tube to the pleura, peri- 

 cardium, aorta, vagi, and thoracic duct are important in the ulceration of 

 malignant disease and infected bodies, and in the passage of instruments. 



The aorta reaches the left side of the vertebral column, with its arch just above 

 the fourth thoracic spine, and thence descends on the front of the column, with a 

 slight tendency to the left, to bifurcate opposite the fourth lumbar spine. 



The spinal cord. — This, about 45 cm. (18 in.) long, extends from the foramen 

 magnum to the junction between the first and second lumbar vertebrae. Up to 

 the third month of fcetal life it reaches to the sacral end of the vertebral canal; 

 later, o^^dng to the more rapid growth of the bony wall, its lower limit is at birth 

 opposite the third lumbar vertebra. The dura mater is continued, as a sheath, 

 as low as the second sacral vertebra. It is anchored above to the upper cervical 

 vertebrae and the foramen magnum, and below, as the filum terminale, to the peri- 

 osteum of the coccyx. The deficiency of the spinous processes and laminae of the 

 fourth and fifth pieces of the sacrum allows of infection, e. g., of a bed-sore reach- 

 ing the membranes, and so the cord. The arachnoid and pia of the cord are 

 continuous above with those of the brain. 



The parts of the column most exposed to injxiry are the thoraco-lumbar and cervico-thoracic 

 partly because here more mobile parts are joined to those which are more fixed, and also from 

 the amount of leverage exerted on the thoraco-lumbar region, and, in the case of the upper 

 region, because this is affected by violence exerted on the head. The chief provisions for pro- 

 tection of the cord are the number of bones and joints which allow of movement without serious 

 weakening, the three curves and columns, cervical, thoracic, and lumbar, ensuring bending 

 before breaking; the large amount of cancellous tissue and the number and structure of the 

 intoryertcbral discs all tending to damp vibrations; the large size of the theca vertebralis and the 

 wa.y in which the cord, anchored and slung by the thirty-one pairs of nerves and the ligamenta 

 denticulata, about twenty in number, occupies neutral ground in the centre of the canal as 

 regards injury directly and indirectly api)liod. 



In lumbar puncture (Quincke) as a means of diagnosis or of relieving pressure 

 advantage is taken of the fact mentioned above that the theca extends below the 

 cord. 



A line drawn joining the highest points of the iliac crests crosses the fourth lumbar spine. 

 Tlie needle is inserted in the median line l)etween tlie third and fourth or the fourth and fifth 

 si)inos, anfl directed forward and slightly ujjward. 'I'he back must be flexed as fully as possible 

 in order to widen the interspinous spaces. Tiie needle is ])aKscd to a depth of about 5 cm. (2 in.) 

 in an adult, or 1.8 cm. (J in.) in an infant. In the supine j)osition the lowest part of the sub- 

 arachnoid space is in the mid-thoracic region, and an anajsthetic fluid, non-diffusible and of a 



