Another example of the pressure of the atmosphere has been slightly mentioned, when treating of the ball-and-socket joint. This is the joint by which the thigh-bone is attached to the hip. As the rounded head of the thigh-bone fits exactly into the cavity of the hip, and is, moreover, well lubricated with the animal oil called synovia, no air can obtain admission between the two. Consequently, they are held together so firmly by the pressure of the atmosphere, that they retain their places even after the whole of the muscular attachments have been removed. Not without very great force can the thigh-bone be dislodged from the shallow socket in which it lies; but, if a hole be bored so as to admit the air, it comes out at once.
Similarly, however firmly a limpet may cling to the rock, if the finest needle were introduced so as to admit air, the creature could not retain its hold for a moment.
The last figure on the illustration represents the common Lampern (Lampetra fluviatilis).
The mouth of this little fish is formed on the principle of the sucker, and very firmly it can adhere, as I can state from much personal experience. Indeed, it is rather alarming, to those who are unacquainted with the character of the fish, to have it turn round and fasten upon the hand. However, it is quite harmless, and those who are accustomed to them will have half-a-dozen hanging on their hand at a time, and take no notice of them.
Already has it been mentioned that Surgery has pressed into its service the weight of the atmosphere by means of cupping. She also makes use of Nature in a similar manner by employing the Leech for local and surface bleeding.
The mouth of the Medicinal Leech forms an exact parallel with the cupping-glass and lancets, only that it is very far superior in its powers. To make the analogy perfect, the lancets ought to be within the cupping-glass, and the latter ought to be able to exhaust the air from itself, and to be attached to a reservoir into which the blood could be passed.
I need hardly mention that the action of sucking as practised by the young of all mammalian beings, from man downwards, is due to the same principle. By the action of sucking a partial vacuum is formed, and the pressure of the atmosphere upon the breasts forces the milk into the mouth of the young.
We might multiply examples ad infinitum, and we will therefore pass to another subject.
Seed-drills.
