44 IXTR OD UCTION. 



with the trachea. The left lung is reduced to give place for the names of 

 the cavity. 



Each pleura is seen to be a closed sack, which lines the corresponding 

 side of the thorax to form the ectal or parietal lamina, and is reflected upon 

 the viscera to form the ental or visceral lamina. Between the heart and 

 the sternum, on the meson, the two pleurse are in contact, and form the 

 septum mediastinale. 



For the sake of simplicity, the pericardium is omitted, and the heart 

 shown as a single cavity with regular walls. The single vessel emerging 

 from the heart will be referred to in 91. / 



The spaces on either side, between the skin and the osseous walls of the 

 coalum and Canalis neuralis are occupied by the muscles, etc. 



The arms have their elbows directed dorsad, and the palms ventrad. 

 The caudal aspect is seen, the bones including the ulna and the ulnare (un- 

 ciforme), and the minimal metacarpal and phalanges. 



90. Relative Positions. The letters A-L are introduced into the 

 diagram for the sake of illustrating certain common cases of designation of 

 relative position. 



The point A is at the meson on the dorsal aspect of the body ; its posi- 

 tion may be described as dorsimesal. B, C, and D are lateral in position, 

 or, to be more specific, sinistral. 



Eelatively, B is laterad of A, while C is dorsad of D. Since they are 

 iipon a curved surface, B may be described as either dorsad or mesad of C ; 

 while C may be said to lie either ventrad or laterad of B. Which expres- 

 sion should be employed would depend upon the context ; in either case, 

 practically, there is no room for ambiguity. 



Ectal and Ental. Of the three planes represented by C, E, F, the 

 former is relatively ectal, the last is ental, and the third intermediate. 



Proximal and Distal. G is at the proximal end of the humerus, and 

 H at the distal end. G is thus proximad of H, and H distad of G. 



Central and Peripheral. On the artery, I is centrad of K, and K is 

 peripheral in comparison with I, but centrad of L. 



On the nerve which leaves the myelon, M and N are relatively central 

 and peripheral. 



91. Use of some Terms of Relative Position in a Physiological 

 Sense. Central and peripheral, and sometimes other terms are employed 

 in a physiological rather than in a strictly topographical sense. 



For example, on the nerve the ventral curve is really, that is topographi- 

 cally, nearer the myelon and the meson than N ; but N" would nevertheless 

 commonly be described as centrad of the curve. 



A striking case of this sort is furnished by the recurrent laryngeal branch of the N. 

 vagus. In a part of its course, it curves around one of the great arteries near the heart, 

 and then passes cephalad to the larynx. Now the larynx is much nearer the brain, where 



