214 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Anatomy of the contractor muscle of the pouch of G. buksaeius. 



As is well kuowu, the pouch of this mammal is simply a pocket- 

 shaped duplicature of the skin of the sides of the head and neck, ex- 

 tending well back over the shoulder, and with its aperture surrounded 

 by a long, narrow, and delicate constrictor muscle. 



The contractor muscle is best exposed by a simple incision extending 

 from the head down the middle of the vertebral colnmn to a point op- 

 posite the hind legs. Care must be taken in making this incision to 

 keep to the median line, especially as the incision is extended back- 

 ward, and to cut through the sMn and superficial fascia only, as other- 

 wise the muscle might be severed near its movable end, which would 

 prove fatal to the study of this portion of it. 



As has been intimated by Professor Ooues in his work before cited 

 in this article, this muscle may be considered a modified form of the 

 platysma myoides, presenting, however, certain well-marked differences, 

 which will become apparent in the course of this description. 



I'or the purposes of this description, I shall divide the muscle into 

 two parts, the first part commencing at the movable head and ending 

 at the inferior angle of the pouch, where the muscle divides -, and the 

 second part commencing at this termination of the first portion, and 

 ending with the insertion of the different heads into the maxillary bones. 



First part. — Commencing at the movable head of this muscle, which 

 is attached by a broad, thin tendon, blending with the tendons of the 

 muscles of the back, covering the last three lumbar vertebrae, and from 

 which it cannot be dissected nor traced beyond, we find that it runs in 

 a nearly straight direction to the inferior angle of the pouch, *. e., the ex- 

 treme portion of the pouch which extends backward over the shoulder. 

 At first it overlies the muscles covering the vertebrae, and afterward 

 it runs nearl3' parallel with, but as it approaches the pouch diverges 

 from, the spinal column. 



The muscle lies in this portion of its course, until near the pouch, 

 under the superficial fascia and in close connection with the skin of the 

 animal above, and beneath with the muscles of the back and shoulder 

 of the animal. As the muscle nears the pouch, it is crossed by a muscle 

 running from the upper and outer part of the shoulder to the ear. This 

 part of the muscle varies from 0.22 to 0.30 of an inch in width, is quite 

 thin, and about 4^ inches in length. 



Second portion. — At the termination of the first part of the course of 

 this muscle, just as it comes in contact with the pouch, it divides into 

 two parts, one of which passes over the upper or outer portion of the 

 pouch, and is inserted into the superior maxillary bone directly below the 

 nose. This part of the muscle is very narrow, and great care is neces- 

 sary that it is not cut away. This band of the muscle lies on the sac 

 below, and is covered by the skin and superficial fascia only. 



The other portion of the muscle passes along the inner and lower sur- 



