The quantity of the insensible perspi ration appears by experiments to be very great. Sanctorius, a Venetian physician, who first noticed its importance and ex tent (whence it has acquired the name of perspirabile sanctorianum) estimated it at five pounds in twenty-four hours, when the solid and liquid food amounts to eight pounds. In temperate climates it may be from two to four pounds daily ; but it varies according to numerous circum stances.
The chief bulk of insensible transpira tion and of sweat is water ; it holds seve ral salts in solution. Carbonic acid gas is also found in considerable quantity ; and even according to some experiments, azote and hydrogen. An oily or seba ceous matter is secreted from the skin, to preserve the cuticle in a proper con dition of suppleness; hence water is repelled from the naked body, when thrown on it. There are also some vola tile and odorous particles furnished from the same source, in which the peculiar smell of individuals and of nations re sides.
Sweat seems to be nothing more than an increase of the insensible perspiration produced by augmented action of the cu taneous vessels. Increased temperature and exercise give rise to it ; and it fur nishes the most powerful means of re ducing that augmented temperature, ac cording to the well known frigorific effects of evaporation. Hence the human body has borne a greater heat than that of boiling water, without having its own temperature raised.
Functions of the Brain and Nerves.—The organs of the animal functions, which keep up the connection between the body and the faculties of the mind, and are therefore found only in animated or ganized bodies, may be conveniently di vided into two classes ; the sensorium, and the nerves : the latter including the nerves and their origins from the brain ; the former comprehending the rest of the cerebral organs, by which the offices of the nerves are connected with the more noble part of our frame, the facul ties of the mind ; and which may there fore not unaptly be termed the organ of the mind. For the differences which animals present in a comparison of the proportions of these parts, as well as in the size of the brain altogether, &c. see COMPARATIVE ANATOMY.
The brain, when brought into view by a removal of the cranium, presents a dou ble motion ; it rises slightly during expi ration, and subsides again when the thorax is dilated. This is explained from the temporary obstruction which the re turn of the venous blood experiences when the lungs are compressed. But a
more conspicuous elevation and depres sion of the cerebral surface arises from the impulse of blood into the arteries of the head ; this motion is therefore per fectly synchronous with the pulse, and may be felt in every infant whose fonta nelle is not closed. The quantity of blood received by the brain is very considera ble ; according to Haller's calculation, be tween two-thirds and a half of the whole mass of blood that enters the aorta. This blood is circulated through all the minute and numerous arterial ramifications of the pia mater, before it enters the brain, as it should seem, in order to diminish its impulsive force ; it rises contrary to its gravity, and its conducting tubes have an angular and tortuous course before they branch out on the pia mater ; which cir cumstances augment the retarding effect. Every thing, on the contrary, facilitates the blood's return, and prevenfs venous distension.
The vast and wonderfully complicated vascular apparatus of the brain, and the large proportion of blood sent to the organ, naturally lead us to expect, that this part and the heart are closely depen dant on each other. if the cerebral ar teries be all tied, the animal perishes in stantly. The influence of the heart, essential to the preservation of life, does not seem to consist so much in the agita tion which the cerebral arteries commu nicate to its substance, as in the effect which the arterial or oxygenized blood exerts on it. For if venous blood be sent into the head, instantaneous death en sues ; and this seems to be the way in which the cessation of respiration, by drowning, hanging, &c. proves fatal.
Nerves, which arise from the brain and Spinal marrow, and are the organs con veying the impressions of external ob jects to those parts, are not found in all structures of the body. The cellular sub:. stance, cuticle, veto mucosum, hair and nails ; cartilages, bones, teeth, perios teum, and marrow; tendons, aponeuroses, and ligaments ; membranes, as the dura mater, pleura, pericardium, peritoneum, &c., the cornea, &c., the absorbent system, the secundines and umbilical chord, are in this predicament. For an account of the disputes which have arisen concern ing the sensibility of these parts see the introductory remarks on sensibility and contractility.