45 



CHAPTER II. 



THE FIBROUS MEMBRANES AND MUSCULATURE. 



The Fibrous Membranes. 



All tlie bones of the skeleton now described are bound together by strong fibrous 

 membranes. In the median plane a tough membrane of this kind extends from bone to 

 bone everywhere except in the median visceral cavity, so that the whole body of the fish, 

 excluding the region of this cavity and the ventral region of the head, is divided into 

 two lateral similar halves by a continuous partition consisting of a tough membrane 

 supported by the bony framework. The structure of this partition in fact resembles 

 that of one of the screens often used in our rooms to keep off draughts, the bones 

 corresponding to the wooden framework, the membrane to the canvas stretched over 

 it. But there is in the organic structure a more intimate connection between the 

 fibrous membrane and the bony framework. Every bone is clothed externally by a 

 tough fibrous membrane, the periosteum, and this is actually continuous with the 

 membrane which connects the bones together. The whole structure is as it were a 

 continuous deposit in whicli certain parts have been difterentiated by the deposition 

 of calcareous compounds and structural modification to form bone. Yet, tough and 

 strong as the fibrous membrane is in its natural state, firm as it is found to be when 

 the muscles are dissected away in the dead fish, if the fish is boiled for a short time it 

 dissolves, and the bones all fall asunder. The reason of this is that the basis of the 

 material of which the fibres consist is gelatine, and therefore though they retain their 

 strength and structure dui'ing life, or in cold water, when subjected to the action of 

 boiling water their structure is destroyed and they become simply a mass of soft liquid 



.ieiiy- 



The median fibrous membrane is continuous laterally with other membranes which 

 are placed at right angles to it and connected with the vertebrse and interspinous 

 bones. These lateral membranes run between the vai-ious muscles, which are attached 

 to them, and externally become continuous with the derma, or deeper fibrous layer of 

 the skin. The principal of the lateral membranes on each side runs longitudinally 

 along the centre of the vertebrae from the skull to the tail, and extends outwards to 

 the skin, thus dividing the muscles of each side of the body into a dorsal and a ventral 



