CHAPTER IX 

 INTERNAL ORGANS 



Skeleton: skull, vertebral column. Muscles. Alimentary canal: mouth, 

 tongue, stomach, intestine, rectum. Vascular system: heart, arteries 

 and veins, lymphatic system. Kidneys. Air-bladder. 



The parts of a fish so far described have been mostly those 

 which can be made out without any detailed dissection, and 

 in the present chapter the true internal anatomy, including 

 the skeleton, digestive system, circulatory system and so on, 

 may be studied. Space will permit of only a brief survey of 

 the more important of these internal organs, detailed descrip- 

 tions of which will be found in any of the recognised text-books 

 mentioned in the accompanying list {cf. p. 435). 



The skeleton, whether composed of cartilage or bone, may 

 be regarded as the local strengthening of certain regions of the 

 connective tissue (which itself forms a scaffolding pervading the 

 whole body), and has clearly been developed in order to give 

 a general support to the body, to provide a protection for the 

 delicate brain and spinal cord, and to furnish an attachment 

 for the powerful muscles. The skeleton of a fish, like our own, 

 is a complicated structure, and is often referred to as the 

 endoskeleton, in order to distinguish it from the more superficial 

 exoskeleton of scales or scutes: as will be shown later on, 

 however, the line of demarcation between the two is by no 

 means as clear cut as would at first appear. Three main 

 regions of the skeleton may be recognised: skull, vertebral 

 column, and fin-skeleton. The last has been dealt with in a 

 previous chapter {cf. pp. 57-59). The skull itself is made 

 up of two distinct parts : the cranium, enclosing the brain and 

 sense organs; and the visceral arches, including the upper and 

 lower jaws as well as the series of segmented arches supporting 

 the tongue and gills. The visceral arches have been discussed 

 in the chapters devoted to the jaws and gills {cf. pp. 35, 106) 

 and only the cranium need be described here. 



To obtain a clear understanding of the general ground plan 

 of a fish's cranium it is necessary to study it in its most primitive 

 form, and for this purpose that of the common Spotted Dog-fish 



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