ANIMAL MECHANICS. 1 

 By J. B. Haycraft. 



Contents : — Introductory, p. 228 — The Movements and Positions of the Body, 

 p. 228— The Mechanism of Joints, p. 233— The Muscle as a Machine, p. 241— 

 The Centre of Gravity of the Body, p. 257 — Methods of Recording Movements 

 of the Body, p. 264 — Detailed Description of Special Movements, p. 267. 



Introductory. — Animal mechanics is for the most part merely a special 

 branch of ordinary mechanics, and the mechanical engineer would at once 

 recognise that many of the problems to be solved are already discussed in text- 

 books of mechanics. Thus, the properties of the shoulder-joint are already 

 known as soon as we recognise that it is a joint of three degrees of freedom, 

 and of the nature of a ball-and-socket joint. But although we may understand 

 the mechanical principles of the shoulder-joint, the movements which take place 

 there are excessively difficult to follow. In the first place, the joint is in the 

 living siibject embedded in flesh, and we cannot be certain of its position in 

 space ; in the second place, we find in practice that a pure movement at the 

 joint almost never occurs, but that it is accompanied by translations of the whole 

 joint, due to movements of the shoulder-girdle. The study of the movements 

 which actually take place at the joint is therefore a mechanical problem of the 

 greatest complexity, and one which it is at present impossible to study with 

 appropriate exactitude. This is typical of the movements of the limbs in 

 general. There are other problems which a previous training in mechanics will 

 not enable us to follow. For these we require a training in physiology, a study 

 of the properties of muscles, and a knowledge of the action of the nervous 

 system, the initiator and controller of our bodily movements. Our knowledge 

 of the facts relative to muscular activity bearing upon locomotion is largely 

 due to the investigations of the brothers Weber of Helmholtz, and A. Fick. 

 The application of the study of mechanics to the human body is due chiefly 

 to the brothers Weber, H. Meyer, and Marey, and recently, to the work of 

 Braune and Fischer. 



In attempting to epitomise these results we shall proceed synthetically, 

 studying first the mechanical properties of the bones, joints, and muscles, and 

 finally the whole body in its upright position and during progression. 



The Movements and Positions of the Body, described by 

 reference to co-ordinate planes or axes. 



In their description of the position of parts of the body, anatomists 

 have for long been accustomed to speak of these in their relationship with 

 certain planes and axes. Thus, the liver is on the right-hand side of 



1 Professor Haycraft having been prevented by illness from seeing tins article through 

 the press, the proofs were submitted to Dr. Rene du Bois-Reymond, who has carefully 

 revised them, and has supplied several additional paragraphs, which have been incorporated 

 in the article. 



