The knowledge of the absorbent system was further enlarged in the latter end of the 18th century, by the pupils of Hunter and Monro, who discovered that these vessels were not confined to man and quadrupeds. They were seen in crocodiles and geese, by Mr John Hunter ; in several birds, in some fishes, and in the sea urchin, by Dr Monro and his assistants ; in the turtle by Mr Howson, who also discovered them in fishes. Mr Shel don gave the first complete account of the lacteals in 1784 ; and two years after, Mr Cruikshank published the first edition of his dizatomy of the Absorbent Vessels of the Human body; a second edition of which appear ed in 1790, and nearly completed our knowledge of the absorbent system. Still later, the Italian anatomist Alas cagni has added to our information respecting the human absorbents, by publishing the most splendid engravings of them that have ever appeared ; and in our own times the method of injecting these vessels has been much improved by employing, in certain cases, a steel or iron syringe, with a capillary pipe, instead of Sheldon's method of filling them, by the weight of a very long column of mercury, falling from the capillary extremi ty of a glass tube. Dr Barclay has used this syringe with the best effect in injecting the lymphatic glands, where considerable force is required, and where the glass tube would be extremely unmanageable.
The most important works on the absorbent system are, Sheldon's History at the Absorbents ; Cruikshank's Anatomy of the Absorbent Vessels ; Monro de Vasis Lymphaticis Valrulosis ; Hewson's Eiperimental En quirks into the Lymphatic System ; Mascagni Dr Vents Lymphaticis ; and Bichat's Anatomic Generale, torn. ii. On the diseases of the lymphatics, sec a Dissertation by professor Soemmering, De Morbis Vasorum Absor bentiunz Corporis Humani ; Adams's Observations on Morbid Poisons ; and Baillie's Morbid Anatomy. The best figures of the Lymphatics are those of Cruikshank and Mascagni, the latter of which hare been copied into Loder's Tabul.r ?natomicer, and Dr Parr's New London Medical Dictionary. A view of the Lacteals and Me senteric Glands, with their relative situation with re spect to the mesenteric vessels, is given in Plate XIX. of the present work, Fig. 12; and a general view of the principal superficial absorbents, and several of those that arc deep-seated, is afforded by the whole length figure in Plate XX.
The figure in this plate is represented as entire, but for the purpose of displaying more accurately the course of the lymphatics, the skin and the sternal parts of the chest and belly are supposed to be transparent, so that the absorbent vessels may be seen through them. The contents of the chest appear in their natural state, but a great part. of the abdominal viscera, comprehending the intestines, the pancreas, the spleen, and the urinary bladder, are supposed to be removed. The liver, the stomach, and part of the spleen, and the kidneys remain. Numerous superficial absorbents are seen running up the inside of the legs and thighs, especially on the left side, and passing through the inguinal glands into the belly, where they are seen gradually to unite in the space between the two kidneys, with the lymphatics and lacteals from the intestines, forming the thoracic duct, or that white tube which is seen passing along the spine, behind the liver and the heart, and appearing again in the upper part of the chest, on the left side, till it reaches the part where it makes a turn downwards to enter the veins. A great number of superficial ab
sorbents are also seen running up the inside of each arm, especially on the right, where they follow the course of the humeral vein, and pass through glands in the arm-pit and shoulder. Other absorbents are repre sented as coining down the neck, and all entering the thoracic duct.
Of the ORGANS Of RESPIRATION and VOICE.
Respiration, or that function by which atmospheric air is alternately received and emitted by the animal body, is performed by organs vet y different in their uses and structure. Those which are more immediately required for the reception and emission of the air, are the lungs and the windpipe; but these organs appear to be chiefly passive in the function of respiration. Others are ne cessary for the purpose either of expanding the cavity in which the lungs are placed, and thus enabling them to receive a greater supply of fresh air, or for cpmpress fig them into a smaller space, so as to expel a part of the air which had become unfit for effecting the neces sary changes in the animal economy. These accessory organs are chiefly the diaphragm, the ribs, and the muscles, by which these are elevated or depressed. In the present chapter we shall describe the lungs, the wind-pipe and its appendages, and the diaphragm.
Of the General Organs of Respiration.
Before describing the lungs, it is necessary that we explain the form and boundaries of the thorax or chest in which they are contained.
The cavity of the chest is of considerable size, ex tending front the lower part of the neck to tne .scrobtru. Ins cordis or pit of the stomach, on the lure part, to tile last vertebra of the back, behind and in the sides, nearly to the lowest part of the cal Wages of the ribs. It is much wider below than abut e, owing to the gradual ex pansion of the lower ribs. As each rib is between the bodies of the vert•brx R hit which it is connected, and as the sternum or breast-bone, from it.. connection with the ribs, partakes of their motions; the cavity of the chest is susceptible or considerable dilata tion and contraction; and these changes in its ext.( nt are much assisted, as we shall see presently, by the cont•ac tion of the diaphragm, or by the strong action of the abdominal muscles pressing the bow, Is upward. The whole cavity is lined by a very firm fibrous membrane, called the pleura, which we shall presently consider more at large. The principal divisions of the cavity of the chest are those formed on the right and left side by the mediastinum extending, from the sternum to the vertebrae. Of these cavities the right is larger than the left, and both are completely filled by the lungs.