The Pentadactyl Type.
Selected writings of A. Jackson Howe.
The grasp of comparative anatomy is splendidly portrayed in this selection from Professor Howe's writings. Dr. Howe visited museums and zoological gardens, dissected dead animals from the circuses, and made frequent visits to the dissecting room long after his preparatory education was supposed to have been attained. The true student and scholar is never through with even the fundamentals so long as there is an opportunity to add to them. When the writer was Demonstrator of Anatomy a student found a bony anomaly in a human subject, and Dr. Howe, hearing of it, came to the dissecting room and examined it. Mounting a stool, he then delivered an impromptu lecture on the hook-like bony spur and discussed the comparative anatomy of human and animal bones and showed the spur to be a point in evidence of the theory of evolution—that the bony anomaly was but the vestige of a normal conformation in some animals that had persisted in incomplete form in the descent of man.—Ed. Gleaner.
THE PENTADACTYL TYPE.—Medicine is a branch of natural science; in its range it dips deeply into zoology. The anatomy of man does not differ essentially from that of other mammals. The philosophic anatomist finds interesting and instructive material for reflection in the structures of what are called the "inferior" animals. Indeed, it has been asserted by the most distinguished scientists that the human body is best understood by those who compared each part of it, so far as comparisons can be made, with homologous parts in the lower animal forms. And those who are just entering upon such studies will be astonished to find how closely nature sticks to a primitive type or form. If she departs from the typical formula for special or adaptive purposes, there seems to be a disposition to return, fully or in part, to the original and favorite model. This tendency to adhere to a fundamental rule is exemplified in the number of cervical vertebrae in mammals. Man in his comparatively short neck has seven bones, and so has the bat, the porpoise, and other almost neckless creatures; and in the long-necked giraffe, camel, horse, deer, and weasel, there are but seven vertebras—a typical number which prevails with wonderful pertinacity, considering the scope for adaptive variety in the length and functions of the mammal's neck. The only exceptions are in a species of sloth and the tropical manati.
Five is a common number for digital division among vertebrates; and our own hands and feet present these digits in a high degree of perfection. Our fingers, with the opposable thumb, are not equaled in function by the digital development reached by any other animal. Man has been classed alone as bimanous, on account of his possessing two hands. A unity of method in the construction of the carpal and tarsal terminations is strikingly apparent not only in the higher, but in the lower vertebrates, fishes alone forming exceptions. The pentadactyl has a wide range of application—it reaches reptiles as well as walking, swimming, and flying mammals. Divergencies are common, for the necessities of modified organizations enforced variety. The herbivora must have feet and legs suited to their manner of living; and the carnivora need digital terminations which shall enable them to capture and tear in pieces their prey. If the claw be sharp its point is protected by a sheath and by being raised from the ground. Amphibious animals adhere quite closely to the pentadactyl type, though their digits may be concealed by a web. Birds apparently depart considerably from the prevailing order of digital division, yet in their legs and wings may be found the evidence that they are constructed in accordance with the somewhat rigid formula.
Variations of digital termination can not be fully comprehended without considering, anatomically and functionally, all the bones which constitute what are denominated the shoulder and pelvic girdles. In an anterior limb may be found a scapula, humerus, radius and ulna, and carpal and metacarpal bones, to which the phalanges are attached; and the greater the number of digits, the nearer certain it is that a distinct ulna and the usual complement of metacarpal bones will be present. A posterior extremity embraces a haunch bone, a femur, tibia and fibula, tarsal and metatarsal bones, and phalanges; and when five toes are fully developed, as many metatarsal bones exist, and the fibula is present. But if two digits disappear, there is a corresponding shrinkage in the metatarsus, and a dwindling effect manifested in the fibula.
The Simian thumb is not opposable to all the fingers—it is dwarfed and imperfect; and in the inferior animals this digit is the first to shrink and disappear. The fifth or "little finger" is occasionally rudimentary, yet it puts in an appearance oftener than the pollex, or first digit. The third and fourth digits not unfrequently attain gigantic proportions, and usually at the expense of the other digits. The hallux, or "great toe," is functionally important in the monkey, but it is apt to shrink and vanish in the lower animals. The raccoon, which is anatomically allied to the monkey, and exhibits many Simian freaks of character, possesses five digits upon each pedal extremity. The opossum is also pentadactyl, and the hallux is placed at right angles with, and is opposable to, the other four digits; it has a short and thick terminal phalanx that bears no nail. Foxes, dogs, wolves, and hyenas possess four functional toes which reach the ground, and a rudimentary digit of greater or less development which bears a nail, but does not come to the ground, nor have any functional importance; and this dwarfed toe is on the inside of the carpus or tarsus, where the most important digit exists in man.
All the feline race are pentadactyl, yet the first digit in each foot is rudimentary and mostly devoid of function. Minks, otters, and beavers exhibit five digits on each foot; and so do many of the great aquatic mammals. Frogs have five toes behind and four in front, with a knob on the carpus to represent the fifth. The alligator has five digits in front and four behind, with a mark for the fifth. Animals with a lizard-like conformation have from three to five toes; and in some lacertian swimmers the shoulder and pelvic girdles are rudimentary all the way through, the legs being too feeble and undeveloped to sustain the weight of the body.
Herbivorous animals have mostly, for each foot, two strong toes that come to the ground, and two rudimentary digits which are called "dew claws," and have no functional importance. The latter bear diminutive hoofs, embrace phalanges, and have meta-carpal and metatarsal splint bones. The fifth digit in these cud-chewers, or ruminants, is rarely or barely represented by a mark or sign, hence such animals are denominated artiodactyl, or even-toed. Most of them present no upper incisor teeth; and they grind their food imperfectly while it is being cropped and swallowed. Their intestinal canal is long and complicated, for the purpose of extracting nutriment from herbage not always rich in nutritious supplies.
Man possesses a distinct radius and ulna, and a tibia and fibula; and so do most of the perissodactyl or odd-toed animals—those having one, three, or five digits. The pig has upper and lower incisor teeth, ankylosed radius and ulna, and a distinct tibia and fibula. Its toes are like those of ruminants, two functional and two rudimental on each foot.
The sheep, the goat, the ox, the buffalo, the moose, the deer, and the antelope have an ulna with a well developed olecranon process, but the lower extremity blends with the radius; the fibula of these animals is wholly wanting, or is represented by a mere knob on the upper extremity of the tibia. The limbs of turtles and alligators possess a radius and an ulna, and a tibia and fibula, all being distinct and evenly divided as corresponding bones in the limbs of the human race.
The horse is a one-toed creature; but the fossil remains of its extinct predecessors show that the original hippus, or the earliest of the equine family, possessed five toes, and was not larger than the smallest ponies now in existence. The fossil bones of a horse with three toes, the central digit being the largest, are found in the Eocene and Miocene beds of the Upper Missouri River—in the "bad lands" of Wyoming. In the Pliocene strata are found the fossil bones of a bigger horse, which had a large toe that reached the ground, and two lateral toes that were rudimentary, as are the "dew-claws" of an ox or other even-toed animal.
The horse of our time is one of the most beautiful and highly developed of the great animals. He is fleet of foot and strong of limb. Each pedal extremity possesses a series of toggle-joints, as the articulations of the limbs may be called, consequently in him is made the best provision for an outlay of muscular and mechanical power. The legs are long and slender, and moved by muscles which are admirably arranged for the development of strength and speed. The spinous processes of the anterior dorsal vertebrae are long and high, to give an elevated attachment to muscles which indirectly lift the feet from the ground. A horse "high in the withers" is not likely to stumble when he trots. The moose is a trotting animal, and has few smooth roads to travel upon, consequently it is very high in the withers, even higher than the horse. The deer runs by leaps, and rarely trots, therefore it need not be high in the withers, and is not relatively so high in that region as the moose. The humerus of the horse is buried in the flesh of the shoulder, and the femur in the tissues of the hip, so that neither can be traced in the outline of the limb; yet these bones are very large and compact, and so obliquely placed as regards adjacent bones that they afford admirable angles for dissipating jars and shocks. What is ordinarily regarded as the knee in the front and hind limbs is really what in man are the wrist and the ankle. The ulna is prominently developed in the olecranon process, but becomes a splint below, and blends with the radius; the fibula is represented only by a process of bone projecting from the upper extremity of the tibia. The carpus and tarsus of the horse consist of two chains of comparatively small bones, as representative parts do in man; but the metacarpus and metatarsus are wonderfully transformed or differentiated. The central metacarpal and meta-tarsal bones—called cannon bones—are large, long, and strong; and the lateral metacarpals and metatarsals are represented by splints, which can be barely outlined from the upper end of the cannon bones to a point a little below the middle of the great central shaft, which represents about all there is of the metacarpus and metatarsus. And below this is a central continuation of a single row of phalanges, without even splints to represent lateral digits. Five sets of phalanges are consolidated in one row. This consists of the upper pastern, the lower pastern, and the coffin bone, which represents the terminal or ungual phalanx, and supports a hoof instead of a nail, as in man and many other animals.
The horse has long lips to gather in its food, and six good incisors in each jaw to crop grass; behind these are short tusks in the male, then comes a toothless space for the "bit," and still further back are the immense grinders which do such excellent service in mashing and pulpifying the food, whether it be grass, hay, or grain. And the grinding is so well done that the food does not have to be regurgitated and chewed over as a cud.
The elephants foot conforms pretty nearly to the pentadactyl standard, for it terminates in five toes; yet the inner toe (hallux and pollex) is somewhat imperfect, or rudimentary in its fundamental character. The hippopotamus treads upon four toes, and has the rudiment of a fifth, which is on the inside of the carpus and tarsus. The rhinoceros goes upon three toes, the "little toe" vanishing entirely, and the "great toe" existing in a rudimentary state, or it is not developed sufficiently to reach the ground.
The kangaroo has five digits in each of its forepaws, and apparently three, though really four, in each of the hind feet. The failure is on the inside of the foot, the hallux is wanting, and the next two are so dwarfed that in the seemingly combined state they are not equal to the outside digit, which again is smaller than the immensely developed fourth toe, reckoned in the order from "great" to "little." The two dwarfed toes which appear in the living animal as single, have but one metatarsal bone that reaches back to the tarsus; and this is very slender. When the animal sits at rest the os calcis reaches the ground, but in hopping about the two outside toes in each hind foot receive the weight of the body, and break the shock by means of a well developed plantar arch and elastic plantar ligaments which stretch from heel to toe as a cord subtends a bow. The tracks of the kangaroo are much like those of a hopping bird. The bandicoot has a hind foot similar to that of the kangaroo, yet the "little" or outside toe is comparatively dwarfed, and the one next to it is as much more enlarged and elongated—it becomes the greater part of the foot, while the others dwindle to insignificance.
In the fore foot of the mole there is seemingly a violation of the pentadactyl type, for six terminal claws are found. But upon dissecting a limb it is shown that the supernumerary claw is a calciform hook that springs from the radius, and is not therefore carpal except in function—a splint to give width and strength to the spade-like hand.
The sloth has but two toes functionally developed in front; and two rudimentary digits exist, yet the animal is called two-toed. The nails on the developed digits are long, strong, and so curved that when hooked upon the branch of a tree they will not let go, though the animal be asleep or even dead. A species of the sloth has three toes in the fore foot, and a rudimentary fourth. The jerboa has three toes to each hind foot, and three ankylosed metatarsal bones. The conformation of the entire limb is much like that of a bird.
The anterior extremity of the bat has four enormously extended phalanges to give expanse to skinny wings; the fifth digit is only a hook or undeveloped claw. The posterior extremities present five digits to each foot; thus, in the anterior and posterior extremities the pentadactyl type is followed.—HOWE, Miscellaneous Papers.
The Biographies of King, Howe, and Scudder, 1912, was written by Harvey Wickes Felter, M. D.