Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 3: Ins-Pla
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29464/924/
840 
OSSEOUS SYSTEM. (Comp. 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 furculum 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 Vertebrata. 
In the Mammalia again the clavicles are 
much reduced in importance and frequently are 
entirely 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 
scapulæ being here actually placed inside the 
thorax within the ribs, and fixed by ligaments 
to the sides of the bodies of the vertebrae; 
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 trom 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 first 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 öfters 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 scaphoides, lunare, cunéi¬ 
forme, pisiforme, trapezium, trapezoides mag¬ 
num, and uncijorme ; 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
        

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