364 PULMONARY ARTERY. 



terminal branches at the lower third of the leg, descends to the lower 

 part of the interosseous membrane, and emerges upon the front of the 

 ankle, to supply the dorsum of the foot : or the posterior tibial and 

 plantar arteries are large, and the external plantar is continued be- 

 tween the heads of the first dorsal interosseous muscle, to be distri- 

 buted to the dorsal surface of the foot. Sometimes the posterior tibial 

 artery is small and thread-like ; and the peroneal, after descending 

 to the ankle, curves inwards to the inner malleolus, and divides into 

 the two plantar arteries. If in this case the posterior tibial be suffi- 

 ciently large to reach the ankle, it inosculates with the peroneal pre- 

 viously to its division. The internal plantar artery sometimes takes 

 the distribution of the external plantar, which is short and diminutive, 

 and the latter not unfrequently replaces a deficient dorsalis pedis. 



The varieties of arteries are interesting in the practical application 

 of a knowledge of their principal forms to surgical operations ; in their 

 transcendental anatomy, as illustrating the normal distribution in 

 animals ; or in many cases, as diverticula permitted by Nature, to 

 teach her observers two important principles -.first, in respect to 

 herself, that, however in her means she may indulge in change, the 

 end is never overlooked, and a limb is as surely supplied by a leash of 

 arteries, various in their course, as by those which we are pleased to 

 consider normal in distribution; and secondly, with regard to us, 

 that we should ever be keenly alive to what is passing beneath our 

 observation, and ever ready in the most serious operation to deviate 

 from our course and avoid or give eyes to our knife, that it may see 

 the concealed dangers which it is our pride to be able to contend 

 with and vanquish. 



PULMONARY ARTERY. 



The pulmonary artery arises from the left side of the base of the 

 right ventricle in front of the origin of the aorta, and ascends obliquely 

 to the under surface of the arch of the aorta, where it divides into the 

 right and left pulmonary arteries. In its course upwards and back- 

 wards it inclines to the left side, crossing the commencement of the 

 aorta, and is connected to the under surface of the arch by a ligament- 

 ous cord, the remains of the ductus arteriosus. 



Relations. It is enclosed for one half of its extent by the pericar- 

 dium, and receives the attachment of the fibrous portion of the peri- 

 cardium by its upper portion. Behind, it rests against the ascending 

 aorta ; on either side is the appendix of the corresponding auricle with 

 a coronary artery ; and above, the cardiac ganglion and the remains 

 of the ductus arteriosus. 



The Right pulmonary artery passes beneath the arch and behind 

 the ascending aorta, and in the root of the lungs divides into three 

 branches for the three lobes. 



The Left pulmonary artery, rather larger than the right, passes in 

 front of the descending aorta, to the root of the left lung to which it 



