3. Flexor longus pollicis is shorter but stronger than the former muscle. It is si tuated the outermost of the three deep muscles of the leg, in contact with the fibula. It arises tendinous and fleshy from the lower half of the posterior surface and outer edge of the fibula, with the exception of the undermost portion. The fleshy fibres terminate in a tendon which passes behind the inner ankle, through a groove in the tibia ; next through a groove in the astrag,alus ; crosses in the sole of the foot the tendon of the flexor longus digi torum, to which it gives a slip of tendon ; passes between the two heads of the flexor brevis pollicis, and then runs in a sheath of tendinous structure which binds it to the under surface of the phalanx, and is inserted into the base of the last phalanx of the great toe. The relations of this muscle in the leg are, pos teriorly it is covered by the deep fascia, which separates it from the soleus; anteriorly it is in contact with the fibula, and overlaps the tibialis posticus muscle and the peroneal ar tery. Its connections in the foot have been explained above. The action of the flexor longus pollicis is not confined to the great toe ; by means of the slip of tendon, which it gives to the flexor longus digitorum, it acts also upon all the toes, and secondarily upon the foot itself, assisting powerfully in the elevation of the heel in progression. But the mode of action of this muscle, and its complicated rela tions with the other muscles of the foot, are too curious to be passed over with a slight ex amination; in fact, we think it may clearly be shewn that there is here one of the most curious and beautiful arrangements and successions of muscular action to be met with in the whole system. We have elsewhere shewn that, from the peculiar form of the foot, the action of the peroneus longus is essential to transmit the burden of progression from the weaker to the stronger side of the foot. (See article Frio; MUSCLES OF.) Let us now follow on the pro gress of the foot in the act of walking, and we shall readily perceive the succession of action of its different parts, and the functions which each muscle performs. It is evident that the smaller toes being shorter than the large one, and nearer to the heel, they will, in the act of elevating the heel and propelling forward the body, come to their bearing on the ground somewhat before the great toe, their action being, in fact, by the breadth of base which they give to steady the onward progress of the body, and to deliver over accurately and se curely the weight to the great toe, the main organ of propulsion of the body. In order to accomplish this to the best effect, it is neces sary that the succession of actions should be accurate and complete, and that the muscles of the smaller toes should exert themselves be fore that of the great toe. To this end the flexor longus pollicis gives a slip to the flexor of the toes, and by the commencement of its action, which merely firmly plants the great toe against the ground, rouses the muscles of the other toes, assisting them to complete their part of the process, while its own labour continues and is at its height when theirs is necessarily accomplished and at an end. Thus, by a beautiful combination and series of actions, the powerful effort of the great extensors of the foot is controlled and guided to its proper end, first by the peronei, next by the flexors of the smaller toes, assisted by the long flexor of the great toe ; and the body propelled onwards and balanced on this toe, the action is com pleted by the further effort of this one power ful muscle. The economy of muscular power
is here not less striking than the combination of action, for the flexor longus pollicis being inserted into the last phalanx of the great toe, its own proper action is not called for till after the muscles of the other toes have performed their part; this muscle, therefore, considerably the most powerful of all this deep layer, were it not for the simple expedient of the slip of communication to the other flexors, would be comparatively useless until the last moment of the propulsion onwards of the body. But now it lends its powerful assistance to the weaker muscles previous to its own peculiar effort, and when all its power is called for, the collateral demand has ceased.
4. Tibialis posticus is situated on the back of the leg between the last-named muscles. It arises fleshy from the posterior surface both of the tibia and fibula, immediately below the upper articulations of these bones with each other. Between the two portions of this at tachment is an angular opening through which the anterior tibial vessels are transmitted. The muscle also arises from the whole interosseous ligament; from the angles of the bones to which that ligament is attached, and from two thirds of the flat posterior surface of the fi bula. The fibres run obliquely towards a round tendon, which passes behind the inner ankle, through a groove in the tibia. It is here situated close to the bone enclosed in a sepa rate synovial sheath. It is inserted into the tubercle on the plantar surface of the os navi culare, sending tendinous filaments to most of the other bones of the tarsus, and to the meta tarsal bones of the second and middle toes. This muscle is covered at the lower part of its origin by the flexor longus digitorum and flexor longus pollicis, and cannot be seen till those muscles are separated. But superiorly it is covered by the soleus only, and here the poste rior tibial vessels rest upon it. Its anterior surface is in contact with the interosseous liga meat, the tibia and fibula. Its tendon runs close to the inner ankle and tarsal bones, and where it slides under the astragalus, is thick ened by a cartilaginous or bony deposit within its fibres, analogous in force and use to the sesamoid bones in other situations. Its action is to extend the foot upon the leg, and to turn the sole of the foot inwards.
(A. T. S. Dodd.) LIFE.—Few abstract terms have been em ployed in a greater variety of significations, or more frequently without any definite meaning at all, than the one now to be considered. And there is none regarding which it is more essential to possess correct ideas, in order to attain the fundamental truths of physiological science. The prevalence of what we deem very erroneous notions on this subject, will oblige us to follow a different plan in its treat ment, from that which we should have adopted if our duty had been merely to give an expo sition of the present state of our knowledge respecting it. We shall commence by offering a short statement of our own views, in order that we may, in the thief historical summary which it will be proper to include in this ar ticle, more concisely indicate what we regard as the errors and inconsistencies of the prin cipal theories which have obtained credit at various times. We shall subsequently con sider more in detail some of the questions which require fuller discussion.