640 



OSSEOUS SYSTEM. (Cowp. ANAT.) 



ployed in constructing the shoulder-joint, but 

 is by no means constantly present. In Fishes its 

 existence as a distinct bone is not recognisable ; 

 but in all the Reptilia it constitutes a highly 

 important piece of the skeleton. 



In Birds the clavicles, in consequence of the 

 elasticity and strength indispensable in the 

 composition of the bony framework of the 

 shoulder in animals constructed for flight, pre- 

 sent a very peculiar arrangement,being generally 

 solidly anchylosed to each other in the mesial 

 line, where they meet, forming a single bone, 

 to which the name of J'urculum is generally 

 given. In Birds, however, that are not organized 

 for flight, such as the Ostrich, this peculiarity 

 is dispensed with, and two distinct clavicles 

 are found articulated with the sternum, as in 

 the generality of Vertebrate. 



In the Mammalia again the clavicles are 

 much reduced in importance and frequently are 

 entiiely wanting, as in all the pachydermatous 

 races. It is only when extensive movements 

 are required in the anterior limbs, either for the 

 purposes of flight, climbing, digging, or pre- 

 hension, that clavicles are interposed between 

 the shoulder of a quadruped and the anterior 

 portion of the sternum, so as to form a kind of 

 pivot on which the whole shoulder moves, and 

 in the human subject the freedom of motion 

 obtained for the arms and hands by this arrange- 

 ment contrasts strongly with the fixed condition 

 of the shoulder, both of Birds and Reptiles. 



The coracoid bone, forming the third element 

 employed in constructing the shoulder-joint of 

 Vertebrate animals, is only fully developed in 

 the Reptilia and in Birds. In Fishes it is but 

 doubtfully represented by two bony pieces 

 already referred to ; but in all the Batrachian 

 and Saurian Reptiles it constitutes the strongest 

 support of the shoulder, abutting on the sternum 

 on the one hand, and on the other firmly con- 

 nected with the shoulder-joint. In the Chelo- 

 nian Reptiles, too, the coracoids are very large, 

 and remarkable on account of the extraordinary 

 inversion of the skeleton of these animals, the 

 scapula? being here actually placed inside the 

 thorax within the ribs, and fixed by ligaments 

 to the sides of the bodies of the vertebras ; 

 while the coracoid bones, equally placed within 

 the thoracic box, are similarly circumstanced as 

 regards the plastron or enlarged sternum that 

 covers them inferiorly. 



In Birds the coracoid bones are of peculiar 

 strength and solidity, serving as buttresses to 

 Support the shoulder against the vigorous trac- 

 tion of the enormous pectoral muscles. It 

 stretches from the anterior margin of the 

 sternum, with which it is firmly articulated, to 

 the junction of the scapula and clavicle, where 

 it assists in forming the glenoid cavity. 



Throughout all the Mammalia, with the 

 exception of the Monotremata, the coracoid 

 bones are wanting or only represented by a 

 small apophysis, consolidated with the neck of 

 the scapula, as is the case in the human skeleton, 

 to which the term coracoid process has been 

 generally applied. 



The humerus, the fit>t bone of the anterior 

 extremity, is invariably a single bone interposed 



between the glenoid cavity and the forearm. It 

 is invariably present throughout all the Reptilia, 

 excepting of course the apodal Ophidian races, 

 and is at once recognisable by the anatomist. 

 In Birds, likewise, the humerus offers nothing 

 remarkable except the mechanical arrangement 

 of its articular extremities. 



Neither in the Mammalia is there any aberra- 

 tion from the common type of structure, the 

 only variations being in the length, form, or 

 proportions of this piece of the skeleton, adapt- 

 ing it to the necessities of the different races of 

 Mammifera. 



The forearm, or second division of the upper 

 extremity, is normally made up of two bones, 

 called respectively the ulna and the radius. 

 These are incomparably most complete in the 

 human subject, where their admirable connec- 

 tions with the humerus, with each other, and 

 with the hand, are amongst the most striking 

 instances of perfect mechanism met with in the 

 animal creation. 



In Fishes and in the Batrachian Reptiles 

 they are most imperfectly developed, and are 

 invariably anchylosed together. In the Chelo- 

 nian and Saurian Reptiles they become quite 

 distinct from each other, but the movements of 

 pronation and supination are extremely limited. 

 The ulna of Birds is the principal bone of the 

 forearm, while the radius is a separate bone 

 easily distinguishable by the relations it bears 

 to the other parts of the wing ; here likewise, in 

 consequence of the uses of the anterior extre- 

 mity as instruments of flight, these bones are 

 almost immoveably fixed in a state of pronation. 

 In the unguiculate Quadrupeds generally, 

 the ulna and radius are separate bones, with 

 a few exceptions, such as the Cheiroptera, 

 where one bone only constitutes the forearm, 

 but amongst the Ungulata they are frequently 

 more or less consolidated and fused together 

 towards their distal extremities, as, for example., 

 in the Ruminants and in the Solidungula. 



The carpus, forming the third division of the 

 upper extremity, generally consists of several 

 short and thick bones firmly bound together by- 

 ligaments, but allowing of sufficient motion be- 

 tween each other to afford a slightly moveable 

 basis to support the parts composing the hand, 

 either to prevent concussion in walking or to 

 permit increased mobility to the fingers. When 

 most completely developed, as they are found 

 in the human subject, they are eight in number, 

 to which names indicative of their shape have 

 been applied, such as scuphoides, lunure, cunei- 

 J'urine, pisifonne, trapezium, trapezoides mag- 

 num, and iincijorme ; but these names cannot 

 be supposed to be applicable to the carpal bones 

 of other Vertebrata, in which they present so 

 many varieties both in their shape and position 

 as frequently to be quite unrecognisable as the 

 analogues of each other, their number too 

 varying most considerably, either on account 

 of the coalescence of elements originally dis- 

 tinct, or from their total suppression. 



The bones of the carpus in Fishes are gene- 

 rally represented by four or five small pieces 

 interposed between the bones of the forearm 

 and the pectoral fin. With these bones the 



