MUSCLES.
The nature and objects of the present work render it impossible for us to enter into the details of comparative myology ; we shall therefore restrain our remarks to one or two subjects.
The differences which we discern in the muscles of the lower extremity, be tween man and the other mammalia, arise out of that characteristic feature, which so strikingly distinguishes man from all other animals, viz. his erect sta ture. The most minute investigation of this subject will shew us that the erect position belongs to man only ; and there by confirms the elegant observation of the Roman poet : Pronaque cum spectent animalia cetera terram, Oa homini sublime dedit ; calumquelueri Jussit ; a erectos ad sidera tollere =that.
In order to enable any animal to pre serve the erect position, the following conditions are required. • 1st. That the parts of the body should be so disposed as to admit of being maintained with ease in a state of equilibrium. 2dly. That the muscles should have sufficient power to correct the deviations from this state. idly. That the centre of gravity of the whole body should fall within the space occupied by the feet ; and lastly, That the feet themselves should have a broad surface, resting firmly on the ground, and should admit of being in a manner fixed to the earth. All these circumstance: are united in the necessary degree in mar only.
The broader the surface included 133 the feet, the more securely will the line of gravity rest within that surface. The feet of man are much broader than those of any animal, and admit of being separated more widely from each other. The sources of the latter prerogative reside in the su perior breadth of the human pelvis, and in the length and obliquity of the neck of the femur, which, by throwing the body of the bone outwards, disengage it from the hip-joint.
The whole tarsus, metatarsus, and toes, rest on the ground in the human subject, but not in other animals. The simim, and the bear, have the end of the os calcis raised from the surface ; while, on the contrary, it projects in man, and its pro minent portion has a more important share in supporting the back of the foot. The
exterior margin of the foot rests chiefly on the ground in the simix ; which cir cumstance leaves them a freer use of their thumb and long toes in seizing the branch es of trees, &c. ; and renders the organ so much the less adapted to support the bo dy on level ground.
The plantaris muscle, instead of termi nating in the us calcis, expands into the plantar fascia in the simix ; and in other quadrupeds it holds the place of the flexor brevis, or perforatus digitorum pedis, pass ing over the os calcis in such a direction, that its tendons would be compressed, and its action impeded, if the heel rested on the ground.
The extensors of the ankle joint, and chiefly those which form the calf of the leg, are very small in the mammalia, even in the genus simix. The peculiar mode of progression of the human subject suf ficiently accounts for their vastly superior magnitude in man. By elevating the os calcis, they raise the whole body in the act of progression ; and, by extending the leg on the foot, they counteract that ten dency which the weight of the body has to bend the leg in standing.
The thigh is placed in the same line with the trunk in man ; it always forms an angle with the spine in animals, and this is often even an acute one. The extensors of the knee are much stronger in the human sub ject than in other mammalia, as their dou ble effect of extending the leg on the thigh, and of bringing the thigh forwards on the leg, forms a very essential part in the human mode of progression.
The flexors of the knee are, on the contrary, stronger in animals, and are in serted so much lower down in the tibia (even in the simix) than in the human subject, that the support of the body on the hind legs must be very insecure, as the thigh and leg form an angle, instead of continuing in a straight line.