There are two classes which may be conveniently called here Primary and Secondary, the latter being too variable and accidental for further notice. The former lie in three main directions and are longitudinal, oblique or transverse. They represent in graphic characters the nature and degree of the functions exercised by muscles moving the joints which underlie them, and are often called “flexion-lines.” They are “folds so disposed that the thick skin shall be capable of bending in grasping while it at the same time requires to be tightly bound down to the skeleton of the hands and feet, so as to prevent slipping of the skin which would necessarily lead to insecurity of prehension, just as the quilting and buttoning down of the covers of furniture by upholsterers keeps them from slipping. For this purpose the skin is tied by fibres of white fibrillar tissue to the deep layer of the dermis along the lateral and lower edges of the palmar fascia and to the sheaths of the flexor tendons. The folds, therefore, which are disposed for the purpose of making the grasp secure, vary with the relative lengths of the metacarpal bones, with the mutual relations of the sheaths of the tendons and the edge of the palmar and plantar fascia.... The sulci are emphasised because the subcutaneous fat, which is copious in order to pad the skin for the purpose of holding, being restricted to the interval between the lines along which the skin is tied down, makes these intervals project, and these are the monteculi.”[66] [66] A. Macalister, _Palmistry Encycl. Brit._, 11th Edition. This account of them from a leading anatomist shows that not for nothing have these creases been evolved. They are inherited, have an important function and are worthy of study in their humble way: they may be even dignified with the name “character.” They are often double over the joints of the fingers and toes, but, from the functional point of view and for simplicity, may be reckoned as single. Chief Types. [Illustration: Fig. 71.--Flexures on palm of right hand. Drawing made from impression.] The most common types of them in the hand of man are shown in the example given in Fig. 71. 1. A flexure over each phalangeal joint. 2. A flexure at the bases of the digits. 3. A flexure over the metacarpo-phalangeal joints of D 2, 3, 4 and 5 with an oblique direction, called _linea mensalis_. 4. A flexure over these same joints and oblique in direction, but nearer to the wrist--the _linea cephalica_. These flexures 3 and 4, though arising from the flexion of one set of joints should be looked at as separate folds because of their time-honoured popular names. 5. A curving flexure surrounding the thenar eminence, extending from the centre of the wrist along the palm and terminating at the radial border. 6. Variable longitudinal and oblique flexures not specified, which I have called secondary. [Illustration: Fig. 72.--Foot of common squirrel.]