DISSECTION OF THE SHARK 17 



reflex systems (see Figs. 2 and 6, Section 23, and Herrick, 15, 

 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 phys- 

 iological 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. 



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 ampullse of Lorenzini 

 and the lateral line canals. The former are arranged irregularly; 

 the latter comprise four tubes embedded in the deep layer of the 

 skin: supra-orbital, infra-orbital, byomandibular 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 specimen 

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

 the feathery gills, the firm gill rakers, the mode of attachment of 

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



