tiguous parts. Some of these are, to a certain degree, instinctive, being directly subservient to some useful purpose in the animal oeconomy, while they are more or less independent of the will, such as sneezing and coughing. There are others, on the contrary, which are entirely under the control of the will, depending upon the contraction of the diaphragm or the muscles of the chest, which we call into action and regulate at pleasure, like other voluntary actions, such as suck. ing and straining. Some of these actions may be regarded as modifications of the voice, being characterized by distinctive sounds, essentially connected with their final cause, as laughing and weeping. The mechanical actions connected with respiration, which are enumerated by Haller and Sommering are the following; sighing, yawning, sucking, panting, strain. ing, coughing, sneezing, laughing, weeping, hiccup, and vomiting."

Sighing consists in a full and protracted inspiration, by which the cavity of the chest is considerably augmented; its final cause appears to be to promote the passage of the blood through the pulmonary vessels and to enable the air to act more fully upon it. Yawning also consists in a full, slow, and long inspiration, but it differs from sighing in being followed by a slow and full expiration; it is also s El. Phys. viii. 4. 30..40.

Corp. Hum. fab. t. vi. p. 79. § 80..90.

7 Of this list Blumenbach omits sucking, panting, straining, and vomiting. Physiol. Sect. 9. § 162. p. 92, 3. He only enumerates those which he regards as modifications of the voice. See also Sprengel, Instit. Med. t. i. sect. 4. § 220..5. p. 490,.7.

attended by an involuntary opening of the jaws, by which the air has a more free admission to all parts of the chest. In sucking we apply the lips closely to the vessel containing the fluid, and by making an inspiration, we encrease the capacity of the chest; the air in the mouth and fauces thus becomes. rarefied, and the pressure of the atmosphere causes a portion of the fluid to enter the mouth. Panting consists in a succession of alternate quick and short inspirations and expirations, and thus produces a frequent renewal of the air in the lungs, in cases where the circulation is unusually rapid, or where, from some obstruction in the chest, we require a more than ordinary supply of fresh air. In the act of straining, we commence by a full inspiration, and retain the air in the chest, while, at the same time, we contract the abdominal muscles. By this means we not only compress the viscera, and expel their contents, but the flow of the blood is retarded, and it has a tendency to accumulate in the venous part of the circulation. The act of straining enables us to exercise the greatest degree of muscular power, because the trunk becomes firmly fixed and serves as the point in which the actions of all the muscles are centred. Coughing is produced by a quick and forcible contraction of the diaphragm, by which a large quantity of air is received into the chest; this, by a powerful and rapid contraction of the abdominal muscles, is propelled through the trachea with considerable force, and in this way dislodges mucus, or any other extraneous substance which irritates the part. When the irritation is considerable, it is in

voluntary, although, in other cases, it is under the control of the will. Sneezing, in many respects, resembles coughing, but it differs from it in being more violent and in being involuntary. The irritation is applied to a more sensible part, the inspiration with which it commences is more deep, and the succeeding expirations are more violent, and are directed through the cavities of the nose. The final cause of sneezing is obviously for the purpose of removing any irritation from these passages, and by means of the interesting observations of Mr. C. Bell, to which I have so frequently referred, we are able to trace the nervous communications which connect the mucous membrane of the nose, with the muscles that are concerned in respiration, but there is still some difficulty in explaining the physical causes of coughing and sneezing as distinguished from each other. Laughing is produced by an inspiration succeeded by a succession of short imperfect expirations. Although it may be produced by certain bodily sensations, yet for the most part, it depends upon a mental emotion; the theory of laughter, or the connexion which there is between the action and the causes which excite it, is somewhat obscure. The action of weeping is very similar to that of laughing, although its causes, both corporeal and mental, are so dissimilar. It consists in an inspiration, which is succeeded by a succession of imperfect expirations. Both laughter and weeping are supposed by many physiologists to be confined to the human species; but we

See particularly the observation in Phil. Trans. for 1822, p. 287. et seq.

may observe approaches to them in some of the animals which, in other respects, exhibit the most intelligence and sagacity. Hiccup is a quick, involuntary, convulsive contraction of the diaphragm, occurring at intervals, and produced by irritation of the cardiac extremity of the stomach, the gullet, or other neighbouring part. The only remaining action which is connected with the respiratory organs is vomiting, but as this involves some physiological considerations, which are connected with the functions of the digestive organs, it will be more properly considered in a subsequent part of the work.

§ 7. Of Transpiration.

As the functions of the skin, and the action which it exercises over the animal economy, are generally supposed to bear a considerable analogy to those of the lungs, it may be convenient to introduce in this place an account of the cutaneous transpiration. It seems to have been a very early opinion among physiologists, that besides the visible matter of perspiration, a species of invisible vapour is likewise discharged from the surface of the body, and that this discharge is connected with some of the most important operations of the system. The first person who endeavoured to ascertain the amount of this vapour, or, indeed, who may be said to have adduced any very unequivocal proofs of its existence, was Sanctorius. He devoted his almost undivided attention to this subject for the greatest part of his life, and alIbid, xii. 2. 10.

Haller, El. Phys. xii. 2. 4.

9

though we shall probably be inclined to think, that the information which he obtained from his researches was, by no means, proportionate to the labour which he bestowed upon them, yet so preeminent was he above his contemporaries, as an inquirer into the operations of the living body, by the mode of experiment, that he obtained the highest degree of celebrity.

The method which Sanctorius adopted to measure the quantity of the insensible perspiration, as he termed it, was to notice accurately the food that was received into the body, and all the discharges that proceeded from it; the former of these quantities was found, in all cases, very considerably to exceed the latter, and this excess was supposed to be transpired from the skin, in the state of invisible vapour. By comparing the weight of the body under all the circumstances to which it is exposed, or by which its functions are modified, as well in health as in disease, a due allowance being always made for the proportion of the ingesta to the egesta, he endeavoured to ascertain the amount of the insensible perspiration during these different states, and he deduced from them a series of aphorisms, which were supposed to contain the general deductions from his almost innumerable experiments. It is, however, not a little remarkable, and certainly much to be regretted, that he no where gives us any exact numerical account of his results,

2 Medicina Statica; this celebrated work appears to have been published in the year 1614, at Venice, "unde," as Haller observes, " innumerabiles editiones prodierunt."