DISSECTION OP THE SHARK 17 



23, and Herrick, '18, Fig. 68). The somatic sensory and 

 motor systems enable the animal to react appropriately to 

 external stimuli; the visceral sensory and motor systems effect 

 the internal adjustments of the body, such as swallowing, 

 respiration, digestion, etc. 



The fish brain can be clearly seen to owe its form to its physio- 

 logical connections with peripheral organs. We shall see that 

 similar functional factors are present in shaping the form of the 

 human brain, though much obscured by the elaboration of 

 higher correlation centers in the thalamus, cerebral cortex, etc. 

 In most respects there are instructive resemblances between 

 the adult fish and the human embryo at an early age when gill 

 pouches are present (see Herrick, '18, Figs. 68, 69 and 70). 



6. Examine carefully the external form of the head, noting 

 particularly the disposition of the sense organs and apertures 

 mouth, nostrils, gills, spiracles (vestigial gills), endolymphatic 

 ducts (two minute apertures near the midline between the 

 spiracles, by which the internal ears communicate with the 

 exterior) . Notice numerous small pores distributed in the skin. 

 These are the openings of subcutaneous sense organs, which are 

 found only in fishes and amphibians and are termed lateral line 

 organs. There are two series of these, the ampullae of Lorenzini 

 and the lateral line canals. The former are arranged irregu- 

 larly; the latter comprise four tubes embedded in the deep 

 layer of the skin: supra-orbital, infra-orbital, hyomandibular 

 lines, and the lateral line of the trunk (see Fig. 4). On the 

 lateral line canals see further in Section 18. 



7. First open the pericardial chamber by a medial ventral 

 incision from the lower end of the specimen forward to the 

 lower jaw. Note the two-chambered heart, the ventral aorta, 

 and the branches of the latter to the gills (aortic arches) . Now 

 complete the ventral incision dorsalward through the lower 

 jaw and floor of the pharynx, opening up the entire length of 

 the mouth cavity back to the esophagus, and spread laterally 

 the ventral walls of the pharynx to expose the inner surfaces of 

 the gills. Next dissect off the skin of the right side of the speci- 

 men in the gill region, in each gill noting the cartilaginous gill 

 arch, the feathery gills, the firm gill rakers, the mode of attach- 

 ment of the gills to the skull, and the clefts between the gills. 



2 



