SHOULDER JOINT (NORMAL ANATOMY). 



section of it impermeable to light, unless it be 

 saturated with Canada balsam, which then 

 very commonly enters the tubuli, and prevents 

 them from being readily distinguishable. The 

 purpose of the extraordinary density possessed 

 by the extremities of the claws, is evidently to 

 adapt them to the various mechanical uses to 

 which the animal applies them : and it is in- 

 teresting to see that this is attained without 

 any variation in the organi: structure of the 

 parti but merely by a more intimate union, as 

 it would seem, of the solidifying mineral 

 matter with the organic basis. It does not 

 seem improbable that the phosphate of lime 

 which is known to be present with the carbo- 

 nate in the shells of Crustacea, may exist in 

 larger proportion towards the extremities of 

 the claws than in other parts of the shell ; a 

 question well worthy of chemical investigation. 



The periodical exuviation of the shell does 

 not appear to be common to all Crustacea; 

 for, according to Mr. Couch*, it does not 

 take place in many of the sessile-eyed tribes, 

 whose cases are as dense as those of the pe- 

 dunculate orders. It is much to be desired 

 that careful observations should be made on 

 the formation of the new shell in the Crab ; 

 since these would probably throw light on 

 much that still remains obscure in the de- 

 velopment of dentine. 



[The author of the forgoing article is de- 

 sirous that it should be understood that all 

 the statements contained in it, except such as 

 are expressly made on the authority of others, 

 are the result of his own observations ; the 

 general facts regarding the organic structure 

 of the shells of Mollusca, Echinodermata, and 

 Crustacea, having been determined by him in 

 the year 1842, and embodied in a paper read 

 before the Royal Society, Dec. 22 of that 

 year, of which the first of the memoirs cited 

 is an abridgment ; and the subject having 

 been subsequently worked out by him in de- 

 tail, with the aid and encouragement of the 

 British Association, to the reports of which 

 he would refer the reader who may desire 

 additional information as to the results of his 

 researches.] 



BiBLiOGhAPHY. Carpenter, On the Microscopic 

 Structure of Shells, in Annals of Natural History, 

 Dec. 1843 ; and in Reports of British Association 

 for 1844 and 1847. Bowerbank, On the Structure 

 of the Shells of Molluscous and Conchiferous Ani- 

 mals, in Transact, of Microscopical Society, vol. i. 

 London, 1844. G. Valentin, Anatomic du Genre 

 Echinus, in Monographies d Echinodermes vivans 

 et fossiles, par L. Agassiz : Neufchatel, 1842. Lavalle, 

 Reeherches d'Anatomie Microscopique sur le test 

 des Crustaces Decapodes ; in Annales des Sciences 

 Naturelles, Juin, 1847. 



(W. B. Carpenter.} 



SHOULDER JOINT (NORMAL ANA- 

 TOMY OF), f The scapular and the axillary 

 regions are each limited externally by the 



* Report of Cornwall Polytechnic Society, 1843. 

 f This article includes the surgical anatomy of 

 the scapulo-humeral articulation. 



571 



region of the shoulder-joint ; the latter also 

 unites the two former regions to each other. 



The region of the shoulder joint (le moignon 

 de Tepaule} exhibits a rounded projectipn, 

 due to the angle formed by the union of the 

 arm with the shoulder ; and to the surgical 

 anatomist it possesses extreme interest, be- 

 cause its skeleton is formed by the shoulder- 

 joint. 



Some difficulty arises in assigning to this 

 surgical region precise limits. Anteriorly, it is 

 separated from the pectoral region, by the 

 narrow space between the deltoid and the 

 great pectoral muscles (the coraco-deltoid 

 groove, Velpeau) ; above, it is limited by 

 the convex projection of the acromion pro- 

 cess, and by the outer end of the clavicle ; 

 posteriorly, it is confounded with the scapular 

 region ; whilst inferiorly, it extends as far as 

 the insertion of thefo/ds of the axilla. 



The elements of which this region is com- 

 posed, are the following : under the super- 

 ficial coverings lie the deltoid muscle (the 

 greater portion of which belongs exclusively 

 to this space), and in it and beneath it, the 

 branches of the circumflex arteries, and of the 

 great circumflex nerve ; still deeper are situ- 

 ated the exteriorof the capsule of the shoulder- 

 joint, the neck and tuberosities of the humerus, 

 the acromion and roracoid processes, with the 

 attachment to them of numerous muscles 

 and ligaments. 



In this article it is proposed to notice, first, 

 the structures in the scapulo-humeral region 

 which are superficial to the joint ; and, se- 

 condly, to describe the anatomical characters 

 01 the shoulder joint itself. 



In removing the integuments and subcuta- 

 neous layer of areolar tissue which covers the 

 deltoid muscle, the anatomist brings into view 

 numerous small branches of nerves from the 

 cervical plexus (supra-acromial twigs), some of 

 the fibres of origin of the platysma myoides, 

 and some small venous branches which, after 

 anastomosing freely with one another, termi- 

 nate in the cephalic or axillary trunks. 



By the removal of its investing fascia, the 

 deltoid muscle is next exposed : in its origin it 

 corresponds accurately to the insertion of the 

 trapezius ; hence thesdtwo muscles are direct 

 antagonists of each other. The fibres of the 

 deltoid muscle arise from the anterior edge of 

 the outer third of the clavicle and of the 

 acromion process, and from the lower margin 

 of the spine of the scapula : from this extensive 

 line of origin, the fibres in descending con- 

 verge to the humerus, and are inserted on 

 the outer aspect of that bone into a rough 

 surface called the deltoid impression. The 

 insertion of the muscle is outside the limits of 

 the scapulo-humeral region, and belongs to 

 that of the arm, whilst its posterior portion is 

 contained in the scapular region ; so that the 

 anterior, upper, and central portions of the 

 deltoid alone belong to the region under con- 

 sideration. Immediately beneath the clavicle, 

 the anterior edge of the deltoid is separated 

 from the pectoralis major by a triangular in- 

 terval, of which the base, placed superiorly, 



