BONES, MUSCLES AND NERVES,— THIRST. 17 



preopercle (30) articulating witli the tympano-mandibular arch of the skull, 

 or the hyomandibular (23) and the symplectic (31). When remarking upon the 

 form of the opercular bones it must not be overlooked that three, 28, 42, 33, at 

 their edges, often more or less overlap one another. 



The scapular arch which supports the pectoral fin is joined to the occipital 

 bone, and contains the following bones commencing from above : the supra- 

 scapular (46), articulating with which is the scapular (47), next the coracoid (48), 

 and attached to it is the epicoracoid (49, 60), while it is united below either by 

 suture or ligament to the same bone on the opposite side. To the coracoid (48) 

 are articulated two others, the radius (51), and ulna (52), and two rows of small 

 bones placed between the forearm and the fin, or the carpals (53) and meta-carpals. 



The form of the preopercle varies among individuals belonging to the same 

 species, sometimes the change being merely due to age or sex, but in some races 

 the development of its lower limb is much more pronounced than it is in others 

 obtained from a different locality. This merely shows that in the marine as well 

 as in the fresh-water races there are fish subject to variations in the shape of this 

 bone. The lower limb is very short in the young, elongating with age in some 

 forms, but not so in others ; while an arrest of development may easily take place 

 even on opposite sides of the head of a specimen, which, were this the sole 

 criterion of species, might, and sometimes does, produce forms said to be typical 

 of Salmo trutta on one side of the head, and 8. alius or hrachypoma on the other. 



The hyoid arch which is composed of a central and two lateral portions, is 

 attached to the temporal bones by two slender styliform ones termed stylo-hyoids 

 (29) . The bones along the two branches commencing from behind forwards are 

 the epihyal (37) to which the stylo-hyoids are affixed : the ceratohyal (38) to 

 which the branchiostegal rays are attached at their bases, next two small bones 

 termed basihyals (39, 40) between which the small glossohyal (41) extends forwards 

 to the tongue, while a single bone, the urohyal (42), passes backwards. 



Of the five hranchial arches, four bear gills and one is destitute of them ; 

 bounded externally by the hyoid arch they are attached inferiorly and along 

 the median line to a chain of bones, the basibranchials (53, 64, 65), which are 

 situated above the urohyal (42) and are anteriorly connected with the body of 

 the hyoid. These branchial arches pass upwards, and are attached by ligaments 

 to the under surface of the skull. The three anterior branchial arches are each 

 composed of four pieces of bone, which commencing from their inferior attachment 

 are known as the hypobranchial (67), ceratobranchial (58), and the epibranchial 

 (61). In the fourth arch the epibranchial piece is wanting, and superiorly the 

 more expanded upper piece which generally bears teeth, is known as the superior 

 pharyngeal bone (62) : while the fifth arch is composed of the ceratobranchial 

 (58) only, and known as the inferior pharyngeal, and likewise is furnished with 

 some fine teeth. 



The chief masses of muscular structure are seen in the four great lateral 

 muscles of the body, which are arranged longitudinally, but divided by oblique 

 tendinous bands of a gelatinous character (and which dissolve on boiling) into 

 numerous flakes or semi-conical masses termed myocommas. These four longitu- 

 dinal layers of muscles have the tendinous bands directed much as follows : the 

 upper series passing downwards and backwards, the succeeding layer downwards 

 and forwards, the third downwards and backwards, and the lowest downwards and 

 forwards. These flakes are arched backwards, heing convex anteriorly. The 

 number of tendinous intersecting bands correspond with the vertebree into which 

 they are inserted.* 



The nervous system in these fishes in proportion to that of the general mass of the 

 body is comparatively small, while in the adult the brain does not fill the crani al cavity . 

 As regards thirst, it would seem either to be unknown to these creatures ; or 

 living as they do in a watery medium it may be quenched by means of endosmosis 

 through the skin. Were this not the case it would be difficult to conceive how 

 such a longing could be satisfied while residing in salt water. 



* For an account of the various muscles employed for the movement of the fins, the jaws, 

 eyes, breathing, &c., the student must be referred to treatises on comparative anatomy. 



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