THE PULMONARY PLEXUS. 
179 
them, the blood from the left side of the heart ramifies in innu- 
merable capillary tubes. It is thus brought if not into immedi- 
ate contact with the atmospheric air conveyed hither through 
the bronchial passages, yet sufficiently so for that all-important 
change to be effected in it by which it is arterialized or vitalized. 
The pulsatory action of the heart, and probably the vital energy 
of the capillary vessels, sufficiently explain the constant passage 
of the blood ; but how do we account for the supply and change 
of the air ? The answer would seem to be easy, — by the alternate 
dilatation and subsidence of the lungs. This, however, would 
not satisfy either you or me. What is the cause of this alternate 
expansion and contraction of the lungs? 
The Pulmonary Plexus . — The spinal organic nerve on either 
side having entered the chest, and pursuing its course to its 
ultimate destination, begins to send off numerous minute 
branches, which unite with others derived from the recurrent nerve, 
and form a plexus of nerves at the commencement of the bron- 
chus. Pursuing its course, other and numerous filaments are 
given off which form a complete network behind the roots of the 
bronchial vessels, and penetrate with them into the lungs, dis- 
tributing themselves upon their mucous surfaces, and accompany- 
ing them even to the air-cells ; some branches from the gan- 
glionic nerve, and from various detached ganglia situated in this 
neighbourhood, having united with them. These are the infe- 
rior and superior pulmonary plexus, and by which alone the lungs 
are supplied with nervous influence. 
The Muscles of the Bronchi. — Now, in what way are these 
nerves concerned with or adequate to the dilatation and com- 
pression of the lungs? They ramify upon the mucous membrane 
of the bronchi. What is the structure of the bronchial tubes ? 
Tiedeman says that, in the mammifera, on the mucous membrane 
where the cartilages cease to be visible, a layer of muscular 
fibres, arranged in a circle, is almost always seen. Mr. Sewell 
says that, fifteen years ago, he assured himself that the transverse 
muscle of the trachea of the horse extended into the bronchi, 
in which it assumed a circular form. M. Rigot has lately an- 
nounced the existence of a muscular membrane or coat beneath 
the mucous membrane of the bronchi, the effect of which is the 
subsidence or contraction of the lungs, and that he has traced 
the greater part of the branches from the bronchial (pulmonary) 
plexus to these muscles. On the other hand, M. Girard makes 
no mention of the muscles of the bronchi. Mr. Percivall states 
that the bronchial tubes differ in structure from the trachea in 
having no muscular band ; and I must confess, that although I 
have often sought for it in the horse and in the ox, I have never 
