INTRODUCTORY 27 



this purpose into three separate tendinous branches. In a 

 number of species, however, wherein the ambiens was supposed 

 to be wanting, Dr. Chalmers Mitchell found striking proofs that 

 this muscle had suffered extensive degeneration, but that traces 

 thereof were to be found in the shape of a tendinous band 

 attached to the fibula and sending off three tendinous slips to 

 the perforated flexors. More than this, in a dissection of two 

 specimens of that aberrant bird the Hoatzin, he found in one 

 case no ambiens above the knee, but the, so to speak, dismem- 

 bered, distal end thereof forming a round ligament attached to 

 the fibula, whilst its opposite extremity was split up into three 

 branches, inserted after the manner just described. In the other 

 specimen the belly of the ambiens was present, but the tendon 

 thereof lost itself on the knee. The dismembered extremity 

 thereof was found attached, as in the first-named specimen, to 

 the fibula at the one end, and to the branches of the perforated 

 flexor at the other. Thus there could be no doubt as to the 

 evidence showing that the dissolution of the ambiens was ac- 

 complished by slow degrees, the first stages commencing with 

 the disappearance of the region between the knee and the 

 crossing at the fibula. In a precisely similar way, it may be 

 remarked, the hindmost thoracic ribs disappear, till at last 

 nothing but the head attached to the synsacrum, and the 

 terminal portion attached to the sternal rib next in front 

 remains ; finally, the head also disappears, leaving but a spicule 

 of bone representing the sternal segment of the rib. 



Nervous System and Senses 



Though birds display a high order of intelligence the brain 

 is devoid of superficial convolutions such as are found in the 

 brains of the higher mammals. A slight furrow, however, 

 apparently answering to the sylvian fissure of the Mammalian 

 brain, may be traced in many birds. 



In the size of the brain, however, birds are in advance of 

 reptiles; both the cerebrum and cerebellum being relatively 

 much larger in the former. 



The sense of sight in birds is highly developed ; thereby 

 the soaring vulture detects the presence of food at immense 

 distances, guided largely, no doubt, by the movements of others 

 of its species nearer the feast, though these may be so far apart 



