The sternum is to be soaked in clear water, with a little washing soda to cut the grease, until it is soft, and then scraped the same as the bones of a ligamentary skeleton, which process will be described in the next chapter.
[CHAPTER XXXVIII.]
CLEANING AND MOUNTING SMALL SKELETONS.
The skeletons of small vertebrates should never be macerated previous to mounting, for the reason that their complete rearticulation would be a practical impossibility. The bones must be left united at the joints by their natural ligaments, which when dry become quite hard, and with the aid of either one or two small brass standards will hold the entire skeleton erect and in proper shape. Skeletons mounted thus, with the parts attached to each other by their own dried ligaments instead of wires, are called ligamentous, or ligamentary, skeletons. All mammals smaller than a large fox, all birds smaller than a small ostrich, all turtles, lizards, iguanas, serpents, crocodilians, and all fishes are mounted in this way. Fortunately it is possible to clean to perfect whiteness the skeletons of almost all these subjects without putting them through the maceration process, which resolves everything into its component parts.
Drying before Mounting.—In order to have a skeleton so that it will scrape to the best advantage and become as white as possible, every ligamentary skeleton must be dried before it is finally cleaned and mounted. In a perfectly fresh skeleton the epiphyses and ligaments are so soft the operator would find it hard to keep from destroying them with his keen-edged steel scrapers, and the smaller bones and cartilaginous members would also be in great danger of mutilation in the same way. When a skeleton dries, all these soft portions harden, and when afterward the skeleton is soaked in clear water for two or three days, or longer as may be necessary, the flesh quickly softens so that you can scrape it all away without encroaching on the framework, and the ligaments at the joints are just soft enough that a portion of it may be scraped or trimmed away, and yet leave sufficient to hold each joint together.
Relaxing a Dry Skeleton.—As intimated above, this is accomplished simply by soaking the specimen in clear water until its joints are pliable, and the flesh upon the bones is soft enough to scrape off. In order that the specimen should not become offensive and disagreeable to work upon, it must not soak long enough for decomposition to set in, for that is the first stage of maceration. Therefore, scraping should begin just as soon as the flesh is soft enough to be readily removed.
[a]Fig. 68.]—Steel Bone-scrapers.
Scraping a Ligamentary Skeleton.—The removal of the flesh and other animal matter from a small skeleton is accomplished by scraping the bones with various chisel-edged scrapers specially designed for this work, and by clipping and trimming on the joints with either curve-pointed or straight scissors. The principles to be learned in skeleton-scraping are comparatively few and simple. In the first place, a sufficient quantity of the connecting ligament at each joint must be left to hold the two bones together in proper shape when the specimen dries. This must not be left in a thick, unsightly mass, but requires to be scraped and trimmed down so that it is reduced to as small a quantity as will serve the purpose. In scraping the flesh off the main stem of a bone, such as the humerus, for example, always begin at the end and scrape toward the middle. The skeletons of turtles, lizards, and the like are an exception to this rule by reason of their structure, and should be scraped from the middle toward each end. If you scrape from the middle of a mammalian or avian bone toward either end, before you are aware of it, you have loosened the attachment of the ligament, and have nothing left to hold the joint together. By beginning on the ligament itself, and working away from it, you can scrape it down so thin at the point of attachment that its identity is quite lost, and the point where it ends is hardly visible. This principle applies to the scraping of all ligamentary skeletons, except a few reptiles.