THE LAMPERN AND THE LEECH. 335 



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 r 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 

 I .ampern (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 cup- 

 ping. 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 prac- 

 tised by the young of all mammalian beings, from man down- 



