170 MUSCULAR SYSTEM. 



the astragalus and the calcaneum, and which I have found in 

 the ostrich, in the condition of a large, solid, sesamoid 

 bone. 



The muscular system of the mammalia is greatly diversi- 

 fied by the variety of forms and habits of the animals which 

 compose this class, some of which are organized to move 

 like fishes through the ocean, some to burrow in the earth or 

 to walk on its surface, or to climb on trees, and others to fly like 

 birds through the air. The differences here presented by the 

 muscular system relate therefore chiefly to the organs of motion, 

 which vary according to the nature and situation of the food. 

 The fleshy parts of the muscles in mammalia are generally large 

 and strong, and proportioned to the more massive bones of the 

 skeleton, and the encreased weight of the whole trunk. The 

 respiration being less extensive here than in birds, the tem- 

 perature is inferior, the tendons are less inclined to ossify, 

 the circulation is slower, the muscular fibres are less dense, 

 and their contractions are less energetic, and the muscles 

 compensate for their defective energy by their increase of 

 bulk, which gives a rotundity to the exterior form of the 

 body, and more massive proportions to all the parts of these 

 animals. As the cetaceous animals breathe by lungs, and 

 not by gills, like fishes, the muscles of their vertebral column 

 are disposed chiefly for movements in a vertical plane, and 

 not for horizontal motions, like those of the water-breathing 

 fishes. The great elevation and breadth of the occipital 

 region of the cranium afford an extensive surface of 

 attachment, as in fishes, for the powerful muscles of their 

 short fixed neck, and extended moveable trunk. From the 

 great length of the transverse and spinous processes of nearly 

 all the vertebrae of the trunk, there is a large angular space 

 on each side of the column, for the numerous long and pow- 

 erful muscles which move the caudal vertebrae, and the tail, 

 and the fleshy bodies of these muscles are very indistinctly 

 separated from each other, from the general similarity of 

 their direction and uses. The splenii muscles of the head 

 and neck, the longlssimus dorsi, the sacro-lumbalis, the tra- 

 chelo-mastoideus, the spinalis dorsi, and the complexus, are 

 here especially distinct on the dorsal region of the column, 

 and important in its backward movements. On the lower 

 part of the column is the large quadratics lumborum, and two 



