ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



This phenomenon observed during the development of the Dogfish 



Other vertebrates the conversion of longitudinal ridges into rows 



..-ki-t-valves appears to be the repetition of a process which has 



taken place during the evolution of this phylum, and one readily sees 



the physiological advantage of replacing a complicated muscular and 



nervous apparatus, with its greater liability to failure through pathological 



interference, by an apparatus purely mechanical and automatic. 



Tlu- blood passing forwards from the heart is distributed through- 

 out tin- tissues by a system of vessels to which the early anatomists 

 the name arteries, meaning literally air-tubes, owing to the fact 

 that at death the strongly muscular walls of these vessels commonly 

 ront rat -i and drive the blood out of them so that when opened they 

 contain only air. This expulsion of blood from the arteries at death 

 ; them a characteristic pale colour in a dissection, in striking contrast 

 with the deep colour of the veins which as a rule remain gorged with 

 blood. 



Before sketching out the arrangement of the main arteries in the 

 Dogfish it is advisable to have a clear grasp of the general plan of 

 arrangement of the main arterial trunks in the region of the vertebrate 

 pharynx as these are of particular morphological importance (cf. Fig. 

 120. p. 293). 



(1) Of aortic arches the normal number is six and these are de- 

 nominated according to the visceral arch in which they are situated 

 I. II. Ill, IV, V, VI or Mandibular, Hyoid, First Branchial, Second 

 Uranrhial, and so on. Where the arch is provided with gills the aortic 

 arch is not a wide-open channel throughout but has intercalated in its 

 course the capillary respiratory network. The ventral portion of the 

 an h. supplying this network, is known as the afferent branchial vessel 

 while the dorsal portion which drains the blood from the network is 

 known as the efferent vessel. Of the aortic arches the first (Mandibular) 

 is in most vertebrates a transitory structure in the embryo and soon 

 disappears : the second (Hyoidean) also usually disappears or becomes 

 much modified. 



(2) There is a tendency for the main longitudinal vessels dorsal and 

 \entra! aorta to undergo a process of splitting from before backwards 

 into ri^ht and left halves which with growth become displaced outwards 



remain in comparative proximity to the gill-clefts. This is 

 rly an arrangement for the economy of tissue by diminishing the 

 th of the afferent and efferent vessels. 



[lie paired vessels arising by this process of splitting are con- 

 tinued forwards into the head as the carotid arteries. The prolongations 



