The pleura receives arterial branches from all the principal arteries of the neighbouring parts. Its nerves are small. and not easily distinguished. They are chiefly derived from the great sympathetic and phrenic nerves. In its sound state this membrane possesses little sensibility, but when inflamed, it is extremely sen sible.
In enumerating the organs of respiration. we must not omit the muscles that are employed in dilating or contracting the cavity of the chest, as on the action a these muscles inspiration and expiration immediately depend. In the general table of muscles we have enu merated those of respiration, in the order according to which they most naturally present themselves in dissec ploycd it drawing down the ribs, and thereby contracting that c•tvity. We shall contrast them with each other in the hollowing table : Lion ; but it may be proper, in titi, chapter to bring them together, and to distinguish those which contribute to the dilatation of the thorax, from those which are em The varieties that occur in the organs of respiration, are not very considerable or important, except as they respect the state of the lungs in the and the pre sence or a particular gland in the foetus, which is not found in the adult state.
Before birth, the lungs are much smaller, firmer, and of a much darker colour than after respiration has commenced. They are also of greater specific gravity, and sink in water, except when in a state of putrefaction.
In the foetus there is found a large glandular body, called the thymus gland, situated in the atlantal and sternal part of the chest, between the two plaits of the sternal mediastinum. It covers the atlantal part of the pericardium, and that part of the arch of the aorta from which arise the carotid and subclavian arteries. From this part it rises considerably, so as to occupy a part of the neck. It has four considerable processes ; two cal led its lobes, next the pericardium, which are broad ; and two long and narrow processes called its horns, run ning up the neck. This substance is of a pale red co lour, though it becomes of a darker hue after birth. Its internal structure has not been accurately exami ned. It commonly contains a whitish liquor, but ap pears to have no excretory duct, by which that liquor can be conveyed to any particular part. By Haller and
some other anatomists it is considered as a lymphatic gland, and some have supposed it to be connected with the nutrition of the foetus. It has numerous blood-ves sels from the subclavian and internal mammary arteries ; nerves from the great sympathctics and /tar -vagina, and several lymphatics have been traced passing from it to the thoracic duct.
Qf the Organs of Voice The organs subservient to those modifications of re spiration which we call voice, and speech, are in man more complex than in most animals. They consist chiefly of tic larynx, the tongue, and the lips, with the mus cles and membranes, assisted occasionally in the articu lations of speech by the teeth and the palate. Most of these organs have been already noticed, as connected with the functions of sensation and digestion. It remains for us here to describe the larynx, and enumerate the muscles employed in producing the varieties so remark able in the human voice.
The larynx is a cavity composed of several moveable pieces, joined to the atlantal extremity of the windpipe, and situated in the atlantal and sternal part of the neck. Its general form is not easily described, but in its struc ture it is perfectly regular and symmetrical, being placed exactly in the mesial line, and having each of its lateral divisions equal and similar. From this regularity of structure, it forms a complete contrast with the wind pipe, the form of which is very irregular. The larynx must be considered almost entirely an organ of voice; for though the air passes through it in respiration, a much more simple orifice would be sufficient for the purposes of breathing, as appears in those cases where the opening of the larynx, called glottis, is obstructed, and where breathing may be carried on through an arti ficial aperture made in the wind-pipe below the larynx.
The larynx is composed of several cartilages that are moveable on each other, and connected with membranes that are susceptible of considerable variations in relative position. Views of the larynx and its principal appen dages, are given in Plate XIV. Figs. 5 and 6, to which the references in the following description correspond.