Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/638

Rh 610 M O L M O L forelimb is due to the humerus, which, like the clavicle, is so much reduced in length as to present the appearance of a flattened X-shaped bone, with prominent ridges and deep depres sions for the attachments and origins of the powerful muscles connected with it. Its proximal extremity presents two rounded prominences : the smaller, the true head of the bone, articulates as usual with the scapula} ; the larger, which is really the external tuberosity rounded off, forms a separate synovial joint with the end of the clavicle. This double articulation gives to a naturally loose joint the rigidity necessary to support the great lateral pressure sustained by the forelimb in excavating. The forearm bones are normal, but those of the forefeet are much flattened and laterally expanded. The great width of the forefoot is also partly due to the presence of a peculiar falciform bone, lying on the inner side of the palm and articulating by its proximal extremity with the wrist. Into the radial side and under surface of this bone is inserted a tendon derived from that of the palmaris longus muscle, which, acting upon it as an abductor, separates it from the side of the palm, and so increases the width of the latter, at the same time rendering the palmar integument tense. The muscles acting on these remarkably modified limbs are all homologous with those of the cursorial insectivora, differing only in their relative development. The tendon of the biceps traverses a long osseous tunnel, formed by the great expansion of the margin of the bicipital groove for the insertion of the large pectoralis major muscle ; the anterior division of the latter muscle is unconnected with the sternum, extending across as a muscular band between the humeri, and co-ordinating the motions of the forelimbs. The teres major and latissimus dprsi muscles are of immense size, probably relatively larger than in any other mammal, and are inserted to gether into the prominent ridge below the pectoral attachment ; they are the principal agents in the excavating action of the limb. The cervical muscles connecting the slender scapulae, and through them the forelimbs, with the centre line of the neck and with the occiput are large, and the ligamentum nuchte between them is ossified (as in all true moles) ; the latter condition appears to be due to the prolongation forwards of the sternum (described above) r preventing all flexion of the head downwards ; and, accordingly, the normal oflice of the ligament being lost, it ossifies, and so affords a more fixed point for the origins of the superficial cervical muscles. The skull is long, with slender zygomatic arches ; the nasal bones are strong and early become united, and in front of them the nostrils are continued forwards in tubes formed of thick cartilage, the sep tum between which becomes partially or wholly ossified beneath. There are 7 cervical, 13 dorsal, 6 lumbar, 6 sacral, and 10-12 caudal vertebrae ; of the dorsal and lumbar there may be one vertebra more or less. The sacral vertebra? are united by their greatly ex panded and laterally compressed spinous processes, and all the others, with the exception of the cervical, are very closely and solidly articulated together, so as to support the powerful propulsive and fossorial actions of the Jimbs. Dentition : i. |, c., prm. |, m. |, x 2 = 44 teeth. The upper incisors are simple chisel-edged teeth ; the canine is long and two-rooted ; then follow three subequal conical premolars, and a fourth, much larger, and like a canine ; these are succeeded by three molars with &quot;NY -shaped cusps. In the lower jaw the three incisors on each side are slightly smaller, and slant more forwards ; close behind them is a tooth which, though quite like them, must, from its position in fi out of the upper canines when the jaws are closed, be considered as the canine ; behind it, but separated by an interval, is a large double-rooted conical tooth, the first premolar ; the three following premolars are like the corre sponding teeth above, but smaller, and are succeeded, as above, by three molars. The geographical distribution of the common mole may be said to exceed that of all the other known species of the genus to which it belongs taken together. It extends from England to Japan, and from the Dovre-Fjeld Mountains in Scandinavia and the Middle Dwina region in Russia to southern Europe and the southern slopes of the Himalayas, where it occurs at an elevation of 10,000 feet. In Great Britain it is found as far north as Caithness, but in Ireland and in the Western Isles of Scotland (except Mull) it is altogether unknown. (G. E. D.) MOLECULE IN the conception of the atomic as opposed to the con tinuous and infinitely divisible constitution of matter, it is supposed that portions of matter called atoms exist, which are separated, or are capable of being separated, from each other by empty space. (See ATOM). It may be the case that each atom has unchangeable shape and volume as well as unchangeable mass, but such a conception of an atom is not essential to the hypothesis. It is not even necessary, as explained in the article ATOM (vol. iii., pp. 37, 38), to maintain that no part of space can be in two atoms at the same time. But one attribute of the atom upon which its permanence, or, so to speak, its personal identity, depends, is its constituent mass, and this remains the same, unchanged and unchangeable, through all time. Boscovich, indeed, goes so far as to regard the atom as a mere centre of force, the result of whose existence is that no two atoms or centres can approach each other within a certain distance, while other physicists regard the atomic volume as a distinct portion of space occupied by that atom to the exclusion of every other, and comprising within it matter ideally infinitely divisible, but the parts of which in fact never have been, and never can be, separated from each other. In this latter mode of viewing the subject, all the conclusions of mechanics which are based on the conception of the continuity and infinite divi sibility of matter may be applied to the equilibrium or motion of each individual atom, the atomic theory merely introducing the additional hypothesis that, in fact, these per sistent entities called atoms do exist, and that out of them all substances which affect our senses are constructed. The theory of universal gravitation requires us to believe in the existence of forces or actions between every portion pubis, whereas the true pubic bones are widely separated (as shown at }&amp;gt;). In this mistake he has been followed by most comparative anato mists ; and hence the mole is generally believed to present the unique peculiarity that the outlets of the urinary, generative, and digestive organs do not pass through the arch of the pelvis. of matter and every other portion, determinate in magni tude and direction, and such that, when on the infinitely divisible hypothesis the volumes of these portions are indefinitely diminished, these mutual forces are inversely proportional to the square of the distance between the por tions (the distance between any two points, one in the volume of each portion, being in this case taken as the distance between the portions), and directly proportional to the products of the masses, or quantities of the two portions of matter, such forces being regarded provision ally as ultimate facts, while inviting further analysis and explanation. Chemical and chemico-physical investigations indicate the existence of other actions between portions of matter, following other and for the most part unknown laws, and rapidly becoming inappreciable as the distance between the reacting portions is increased. All these hypotheses are to be retained on the hypothesis of discrete atoms as above enunciated, the mutual actions between atoms being the resultant of the actions between the various portions of their constituent matter. The volumes of the atoms are so small that, for any sensible distances apart, the line of the resultant mutual action between them may be taken as coincident with the line joining any point in the volume of one to any point in the volume of the other, but, for distances or parts comparable with the linear dimensions of the atoms, the size and shape of their bound ing surfaces must be taken into consideration, and perhaps also the law of distribution of their constituent matter within that surface. In all respects, unless we accept the Boscovichian hypothesis, we simply regard the atom as made up, so to speak, of infinitely divisible matter, while substances, as we know them, are built up of indestructible- and unchangeable atoms. With this conception of an atom, as thus explained, we might be content to rest, confessing our total ignorance of the mode in which such atoms are built up into actual substances, being satisfied to regard such substances as.