311 



ARTICULATION. 



ARTOCARPAl 



316 



it U composed of a considerable number of little plates, or facetto* 

 (a* in the fly), each of which receive* a branch from the optic nerve. 

 Such eyes are called Compound Eye*, whilst the Single Eye*, which 

 exist sometimes in conjunction with the compound eye* in the same 

 individual, are called Ocelli. The eye a* an organ of sense in finit 

 "h'tinctiy developed in the Articulate Animals, a* many of the organ* 

 which hare been indicated a* eye* in the Radiate Animals have 

 probably no relation to the function of vision. Some anatomists have 

 described organs of hearing in the insect*, whilt other* regard the 

 nnu-uiue a* destined for the performance of this function. 



The digestire apparatus of the Articttlata is for the most part in 

 accordance with their carnivorous habits. Where animal flesh is eaten 

 there digestion is a leas complicated process than where vegetable food 

 is principally partaken of. The mouth is generally provided with 

 masticatory organs, which move laterally and are provided with palpi. 

 Hard parts subservient also to the function of preparing the food for 

 digestion, are also found in the intestinal cavity. The mucous 

 membrane which cover* the alimentary passages is of the simplest 

 kind, whilst those gland* which contribute to the digestive function* 

 in the higher animals, as the salivary and pancreatic glands and 

 the liver, are either not present or exist only in. the most 

 elementary form. 



The elongated form of the Artifiilata impresses this character on 

 their circulating a* well as digestive apparatus. In most of the 

 articulate tribes the blood move* forward* in one or more large, 

 dorsal, pulsating, arterial vessels. Side branches from these arteries are 

 given off, and terminate in various trunks which convey the blood 

 backward* to the dorsal vessel. The blood is more highly organised, 

 has a deeper colour, containing a larger quantity of corpuscle* and fibrin 

 than in either the Radiate or Molluscous classes. 



The respiration i* effected either by branchiic, as in those which 

 habitually live in water, such as the Crtutafta, or by tracheae, or 

 air-tubes formed of three parts, one membrane internal and one 

 membrane external, both of which are cellular; and a sort of 

 cartilaginous elastic tube, rolled spirally, and placed between the two 

 membranes. These tracheae receive air by certain lateral opening* 

 termed Stiymata. More rarely there exist cellular cavities analogous 

 to lungs. In all instance* the respiratory organs are perfectly 

 symmetrical. 



The following are the families of animal a which are referred to the 

 Articulate Type : 



1. Entosoa, including various forms of animals that inhabit the 

 organs of higher animals. [EXTOZOA.] 



2. Rotifera, or Wheel Animalcules, minute creatures scarcely visible 

 to the naked eye, very abundant in all waters, and fonncrly classed 

 with the Infusoria. [ROTIFERA.] 



S. Annelida, or Annulose Animals, including the Leeches, Worms, 

 and Sea-Mice, mostly inhabiting water. [AXXKMPA.] 



4. Myriapoda, of which the Galley- Worm (lulut) and the Centipede 

 may be taken as types, and which occupy an intermediate position 

 between the highest and lowest forms of the class. [MYBIAPODA.] 



5. Inttcla, in which the locomotive power of the class is most fully 

 developed, nearly all possessing wing* for flight [IKSECTA:] 



6. Criutacea, the insects of the ocean, which breathe by gills instead 

 of trachea;, and include the well-known forms of Lobsters, Crabs, and 

 Shrimps. [CRUSTACEA.] 



7. Cirrhipedia, the Barnacles and Sea-Acorns. They were formerly 

 referred to the Hollusca, but their structure, habit*, and economy 

 place them amongst the Articulata. [CIRRHIFEDIA.] 



8. Arachnula, including Spiders, Scorpion*, and Mite*. They are 

 distinguished from insects by possessing eight legs. Their instinct* 

 and intelligence place them at the head of this class as well a* their 

 structure. [ARACIINIDA.] 



(Grant, Outline* of Comparative Anatomy; Owen, Lecture* on 

 Comparative Anatomy; Jones, Animal Kingdom; Cydopadia of 

 Anatomy and Physiology; Carpenter, Principle* of Phynology.) 



ARTICULATION, the term by which anatomists express the union 

 of the different bones of the skeleton. The junction of any two 

 bones, however firmly or loosely connected, or in whatever mode the 

 union may be effected, is designated by the name of Articulation. 

 Commonly two substances are employed as the media by which the 

 connection is established, namely, a firm and strong membranous 

 tissue termed ligament [LIQAXEHT], which may be considered as the 

 band by which the bones are tied together, and a peculiar substance 

 termed cartilage or gristle [CARTILAGE], which i* often interposed 

 between the surfaces of the bones to be united, and which besides 

 serving as the bond of union, accomplishes other purposes. 



Of all the parts of the animal fabric, there is none in which 

 mechanism is more clearly or beautifully shown than in the connections 

 of the bones with each other, and more especially in the structure of 

 joints. 



The objects to be obtained in the economy by the union of the 

 severs! bones of the body are various and even opposite, requiring 

 almost every conceivable variety in the mode of their connection. 

 And such variety actually exists ; but still these varieties admit of 

 classification, and they may all be arranged under three head*, 

 namely, those which form luimovcablc, Moveable, and Mixed 

 Articulations. 



1. One object to be accomplished by the union of bones is, to form 

 a secure situation for tender and delicate structures. Accordingly the 

 bones are often so disposed as to inclose cavities in which the organs 

 that need protection are placed ; such, for example, i* the ca-. 



the head which incloses the delicate substance of the brain ; the 

 cavity of the spinal column, which incloses the no less delicate 

 substance called the spinal marrow ; and the cavities of the chest and 

 abdomen, which inclose soft and tender organs, on the security of 

 which life depends. Bones forming cavities of this class are generally 

 so firmly united that they admit cither of no motion whatet 

 only "f a very slight degree of it, the union being effected somctmi. 

 by the .apposition of the surfaces of strong and flat bones ; at other 

 times by the formation of numerous prominences and depressions 

 which mutually receive each other : examples of both these modes of 

 union are found in the articulation of the bones of the head and 

 face. The firmness of the union is sometimes increased by alternate 

 indentations and projections, like the teeth of a saw, formed on tin- 

 surfaces of bones, the surface of the one bone being precisely 

 adapted to that of the other ; by this mechanism the bones become 

 firmly impacted, and deficiency in extent of contact is compensated 

 by what may be truly called (and it is an admirable example) dove- 

 tailing. Future i* the term given to this mode of union, and the bones 

 of the cranium are nicely adjusted and firmly united to each other in 

 this manner. At other times a ridge is formed in one bone which in 

 received into a groove fissured in another. The bony |>art of the 

 septum which divides the nostrils afford* a *peciuien of this mode of 

 union, while the teeth are secured in their sockets (that in, a < 

 surface i* firmly impacted in a cavity) very much as a nail i* fixed in 

 aboard. 



2. The Moveable Articulations are those in which the bones ore in 

 contact, but not continuous with each other ; such, for example, is 

 the union of the ami with the shoulder, the fore arm with the arm, 

 the wrist with the hand, the lower jaw with the head, the head with 

 the trunk, and so on. In these cases the articulating surfaces an 

 mutually adapted to each other, in general one being convex and the 

 other concave, and the bone* are maintained in their situation by the 

 firm and strong membrane* termed ligament*. Sometimes the union 

 is assisted by the muscles which surround the joint, as is strikingly 

 exemplified in the shoulder-joint, in which the head of the humeru* 

 i* kept in contact with the cavity which receives it, partly without 

 doubt by ligamentous substance, but portly also by the surrounding 

 muscles. This i* proved by the effect of disease ; for if by paralysis, 

 or any other cause, the neighbouring muscles become very much 

 weakened, dislocation of the joint readily take* place. Both the 

 strength of the joint and the range of it* motion depend mainly on 

 the extent of it* articulating surface, and on the arrangement of the 

 ligainentou* substance by which the bones are held in their situations. 

 The extent of contact, and the strength and adjustment of the uniting 

 band, are different in every different joint, the diversity being regulated 

 in every cose by the kind and degree of motion which it i* intended that 

 the joint should exercise. 



8. The Mixed form of Articulation resemble* the luimovcablc, in 

 having the bones connected by an intermediate substance (cartilage), 

 and the Moveable in admitting some degree of motion between .the 

 surfaces. The articulation* between the several bones that form the 

 spinal column afford examples of this mode of union. There are 

 numerous modifications of these several kind* of articulation, which 

 are described with great minuteness in anatomical book*, and most of 

 which are distinguished by specific names. 



ARTI'SIA (Presl), a Fossil Plant from the Coal Formation. At 

 present the opinion prevails that this is an internal portion of another 

 plant, and not a palm-stem as once conjectured. 



ARTOCARPA'CE^E, Artocarpadi, the Bread-Fruit Tribe, a natural 

 order of plants nearly related to I'rticaccte (the Nettle Tribe), from 

 which it is so difficult to separate them by any precise character that 

 there are many who consider them nothing more than a section of 

 L'rticaceit. 



Whether a distinct order or a section only of L'rticaeett, the 

 A rtocarpacea ore known by having flowers with n very imperfectly 

 formed calyx, no corolla, leave* with conspicuous stipule*, a rough 

 foliage, and an acrid milky juice, which often contains Caoutchouc, or 

 Indian Rulibcr, in abundance ; the flower* are collected into round 

 heads, and the ovule* are suspended singly from the upper part of the 

 Holitory cavity of the ovarium. They are distinguished from the 

 t'rtitacta by the position of their ovules, the manner in which their 

 flowers an arranged, and by their yielding a milky juice ; the juice of 

 i'rticacea: being watery. 



The specie* are all found in the warmer part* of the world, and 

 many of them are natives of the tropics only. Their milk, which i* 

 always acrid, render* some of them intensely poisonous, as the Upas 

 Tree of Java [AXTIARIS], and certain Indian species of Fig [ Fiefs] ; 

 nevertheless, if the milk i* naturally absent from any particular part 

 of an Artocarpad that part becomes eatable and even wholesome. 

 Thus the fruit of the cultivated fig, up to a short period before its 

 maturity, remain* milky, and at that time it would prove exceedingly 

 unwholesome; but when ripe the milk disappears, is replaced by sugar, 

 and the fruit becomes, as we all know, extremely wholesome. The 

 same explanation is probably applicable to the case of the Bread- 



