32 



FIRST CLASS OF THE VERTEBRATED ANIMALS. 



action experiences any mechanical disadvantage, it arise?: from the mode of insertion, 

 and not from the composition of the muscle. This is not the case with the two other 

 kinds of simple muscles, the radiated and penniform. 



The 1-adiated muscles are those which have their fibres disposed like radii of a 

 circle, and which proceed from a base more or less extended, while they incline to- 

 wards each other, and are inserted in a small tendon. 



The penniform muscles have their fibres disposed in two rows, uniting in a middle 

 line, and forming angles more or less acute, so that they resemble in some degree the 

 arrangement of the feathers in a quill. The tendon forms the continuation of this 

 middle line. 



It may be easily perceived that, in the two last-mentioned kinds of muscles, the 

 total or resulting force is less than the sum of the component forces; and that, if we 

 take successively the lengths of every two fibres, which unite in producing one angle, 

 as the measures of their individual forces, the diagonal of the ultimate parallelogram 

 which may be formed thereon will represent the entire resultant, in quantity and 

 direction, belonging to the fibres of the whole muscle. 



When several muscles unite in one common tendon, the result is called a com- 

 pound xiMSG\e. These muscles may be similar in their nature, but sometimes they 

 are formed of very different kinds, such as the radiated and the ventriform uniting to 

 form one compound muscle. We may, then, estimate the particular action of each 

 according to the precedmg observations, and the total action can then be estimated 

 accordin"- to the degree of their inclination. Other muscles, again, are ?,iy\Q A compli- 

 cated : these may have only one belly with divided tendons ; or they may have several 

 fleshy parts, wherein the tendons are interlaced in several ways. 



The absolute force of the muscles is determined from these several dispositions; but 

 it is their insertion which determines their real effect. The muscular insertions may 

 be referred to eight distinct classes: — 1st, the fleshy envelope; 2d, the sphincter or 

 ring; 3d, the curtain; 4th, the rotatory; 5th, the rope; 6th, the lever of the first 

 kind; 7th, the lever of the second; and 8th, of the third kind. The first four have 

 a striking similarity, in their being all formed of a girdle, or portion of a girdle, which 

 contracts upon the surrounding parts. 



1. The diaphragm and abdominal muscles are instances of the fleshy envelope. 

 Beino- destined to compress thesoft parts contained in a certain cavity, they envelope that 

 cavity in every direction, in the form of membranes or bands. When all the fibres act 

 sunultaneously, it is for excretory purposes ; but they usually act alternately, and then 

 the effect is to enlarge one cavity and to diminish the other. Thus, at each inspira- 

 tion the abdomen becomes wider and shorter, while the contrary happens on each 

 expiration. The heart, arteries, and intestines, have muscles of this kind; and the 

 muscles moving the tongue in Man and Beasts must also be referred to this class. 



2. The sphincter muscles are calculated to widen or contract some soft aperture. 

 Some of them surround the orifice like rings, and others are inserted in a manner, 

 more or less directly, upon the edges of the opening. If the muscle be uniformly dis- 

 tributed around the orifice, it always preserves its figure, and is dilated or contracted 

 always in the same manner. But when these muscles have difi'erent directions, and 

 make different angles with the edges they have to move, the form of the aperture is 

 very variable, as we may see in the lips of Man. No animal possesses so great a mobi- 

 lity of this part, and none can therefore possess so expressive a physiognomy. 



3. The curtain muscle is seen in the eyelids of Man and other Mammalia. When 

 these muscles are placed in the body of the membrane, which is destined to cover some 

 other parts, thsir structure is such as we have just described; but when they are 

 situate externally, they have the form of very complicated puUies, as will be explained 

 when we come to treat of the Eye in Birds. 



4. The rotatory motion of the muscles may be seen in the means by which the globu- 

 lar mass of the eye is rolled and supported on every side. 



5. The rope muscle has already been alluded to, in speaking of the larynx, and 

 may be regarded as the most advantageous form in which a muscle can be applied. 



6. 7, 8. When a bone intended to be moved is articulated at any particular point, it 

 cannot be elevated or depressed in a direct Ime, but must be considered as a lever 

 having its fulcrum in the articulation. The bone forms a lever of the first kind when 

 the articulation is between the two extremities, and the muscles are placed at one of 

 them, as we may observe in the muscles attached to the olecranon and heel-bone. 

 But the most usual case is when the articulation is at one of the extremities of the 

 bone ; and then the most favorable position for the muscle is when it rises from 

 another bone parallel to tha*" which it has to move, or which forms with it only a very 

 small angle. This is the case with the muscles between the ribs (intercostales^, and 

 several others. Yet these muscles possess a degree of obliquity which considerably 

 diminishes their power. The muscles closing the mouth of Man may also be com- 

 pared to those just mentioned with respect to their small obliquity; but they are in- 

 serted much nearer to the point of support than the former, a circumstance which 

 also considerably diminishes their force. 



The most usual kind of insertion is where a muscle attached to one bone is inserted 

 into another, which last is articulated either mediately or immediately with the first, 

 and maybe extended until they both form a line, or injected so as frequently to make 

 a very small angle. This mode of insertion appears to be tiie most disadvantageous 

 of all in respect to mere force, on account of the obliquity of the insertion when the 

 moving bone is extended, and also on account of its proximity to the Fulcrum. The 

 first inconvenience is partly corrected by the heads of the bones. Their articular 

 extremities are usually enlarged, so that the tendons of the muscle, by turning round 

 a convexity, in order to be inserted below it, form more obtuse angles with the lever, 

 or body of the bone, than would be practicable if the head did not exist. By this 

 means the obliquity of their insertion is diminished, and rendered less variable. 



Tlie proximity of the fulcrum was necessary to prevent the members from being 

 monstrously large in the state of flexion, but particularly for producing a prompt and 

 complete flexion. As the muscular fibre loses only a determinate fraction of its length 

 by contraction, if the muscle were inserted at a greater distance from the joint, the 

 moveable bone would only be approximated to the other by a small angular quantity. 

 On the contrary, by inserting it near the apex of the angle, a very small contraction 

 occasions a considerable approximation. Velocity is gained in proportion as the space 



through which the muscle acts is diminished. In this manner, muscles of this kind 

 exercise a power which surpasses all imagination. 



There are many instances of muscles inserted at a considerable distance from the 

 fulcrum, especially in the short bones, which must he completely inflected. The ver- 

 tebr£e and phalanges of the fingers are in this situation. Muscles extended from the 

 one to the other of these bones would not have produced a sufficient degree of flexion. 

 In the phalanges, the fingers would have been two thick. It was also necessary that 

 the tendons of these muscles should be attached to the bones over which they pass. 

 If this were otherwise, it would happen that, whenever the phalanges were bent so as 

 to form an arc, the muscles with their tendons would remain in a straight line, and 

 form its cord. We may hence perceive the necessity of the annular ligaments, the 

 sheaths and perforations. The last-mentioned arrangement occurs solely in the flexions 

 of the fingers and toes of Man, Quadrupeds, and some othi-T animals, and consists in 

 the muscles which have to extend farthest being placed near to the bones, while their 

 tendons, perforating those of the muscles, are inserted at a shorter distance, and lie 

 over the first. When there are only three phalanges, there is but one perforation. 

 The muscles moving the tail in the Quadrupeds are placed at a great distance from it; 

 but their long and slender tendons are inclosed in sheaths, which they do not leave 

 excepting immediately opposite the points into which they are to he inserted. 



The whole of the Mammalia have the upper jaw fixed to the skull ; and 

 the lower one is composed of only two pieces, articulated to the tempo- 

 ral bone, by a projecting part [called the condyloid process.] 



By the elongation of the condyles, which fit into the zygomatic process of the tem- 

 poral bone, this joint is nearly restricted to the motions of a hinge, alternately raising 

 and depressing, while the lateral motion is only just sufficient for the grinding of the 

 food. 



There is a single or double bone, found in most Mammalia, called the inter-maxil- 

 lary bone, bat of which Man is entirely destitute. In these animals the upper jaw- 

 bones do not touch each other under the nose, nor do they contain all the teeth, but 

 the inter-maxillary bone is wedged in between the former, and contains the incisive 

 teeth of those animals possessing them. The size of this bone varies surprisingly in 

 the several orders and genera of Mammalia, being small in the Walrus and many Car- 

 nassiers, but large in the Beaver, Marmot, Hippopotamus, and Cachalot, but especially 

 in the Wombat. In the Ornithorynchus it is constructed of two pieces in the form of 

 hooks. This bone is seen to exist in animals altogether destitute of teeth, and is 

 also found in such Ruminantia as have no incisive teeth in the upper jaw. Some ana- 

 tomists have doubted whether the upper jaw-bones and inter-maxillary bones are not 

 the same, and that the latter is merely the anterior or incisive portion of the former. 

 The latter opinion appears to be the more probable, as the division is found in tha 

 human fcetus, while, in some quadrupeds, the two bones are frequently seen to coa- 

 lesce. The lower jaw surpasses all other bones in the variety of its forms among tha 

 difi'erent Mammalia. It possesses very strong projections on the under side in the 

 Wombit ; and we may remark in the Cercopithecus Beelzebub, and other Brazilian 

 Monkeys, a remarkable lateral development of the bone, which assists the larynx in 

 the emission of that extraordinary deafening sound peculiar to these animals. In the 

 Ornithorynchus, the anterior part of the lower jaw is shaped like a shovel. 



An intimate relation may be observed between the kind of food with which an ani- 

 mal is nourished, and the motions performed by its lower jaw; and these again are 

 greatly influenced by the form of its condyles. Thus, Mammalia living on vegetables 

 possess a power of moving their lower jaws from side to side, so as to produce that 

 grinding effect necessary for pulverizing and dividing grain, and for bruising grass. 

 These animals are in this way able to move their lower jaw in almost every direction, 

 by the form of the condyle, and of the cavity to which it is articulated. On the con- 

 trary, with the Carnassiers, we find that the lower jaw is altogether incapable of any 

 other motion than simply downwards and upwards, being destitute of that lateral 

 grinding motion attendant on mastication in its most perfect form. Thus, while the 

 teeth of the Herbivorous quadrupeds may be compared to the stones of a mill, the move- 

 ments of the teeth, or rather tusks, in the Cai-nivorous quadrupeds greatly resemble 

 the dividing motion of scissors. 



The neck consists of seven -vertebras, one species excepted [the three- 

 toed sloth] which has nine. 



A great variety is found in the number of their vertebra;, excepting those of the 

 neck. In the Cetacea, where the neck is very short, the bodies of the Cervical Ver- 

 tebrje are extremely thin, and form by anchylosis one bone ; so that the original number 

 of vertebrse, with their processes, can scarcely be perceived. In Quadrupeds having 

 long and flexible necks, such as the Camel, and Camelopard, the spinous processes of 

 the vertebra of the neck are small, or they are neai-ly obliterated. A pecuhar sub- 

 stance of great strength, called the Ugameiitum nucha:, is attached to the necks of the 

 larger quadrupeds. By means of this elastic body, the great weight of the head is sup- 

 ported. In the Elephant, it is of a very great size. The short-necked Cattle have 

 double transverse processes, and in the bodies of the Cervical Vertebrie, both of Rumi- 

 nating animals and Horses, there is a longitudinal ridge running along the front. With 

 Carnivorous animals, the Ugamentum nuchas is small; and as the pendent position of 

 their heads require strong muscles for their support, the Cervical Vertebrs have their 

 transverse processes very large and fiat, both in the front and back, and thus afi'ord 

 places of attachment for the muscles of the neck, as well as for those which contribute 

 to open their mouths. 



Tiie bngth of the neck does not depend upon the number of the cervical verte- 

 br.8; for, as we have already observed, this is nearly always the same in most quad- 

 rupeds. In general, we find the length of the neck to be such, that, when it is 

 added to the head, their united lengths are exactly equal to the height of the animal 

 from the ground. Were this otherwise arranged, quadrupeds could not easily have 

 reached either the herbs on which they feed, or the water they must drink. The 

 bulk of the head, in all those animals where this rule is observed, is very nearly in 

 an inverse proportion to the length of the neck, else the muscles would be unable to 

 elevate the head. This rule, however, is not adhered to in such animals as Uft their 



